Azo-Functionalized Zirconium-Based Metal?Organic Polyhedron as an Efficient Catalyst for CO2 Fixation with Epoxides

Article abstract of DOI:10.1002/chem.202102089

Chemical fixation of CO₂ as C1 source at ambient temperature and low pressure is an energy-saving way to make use of the green-house gas, but it still remains a challenge since efficient catalyst with high catalytic active sites is required. Here, a novel monoclinic azo-functionalized Zr-based metal?organic polyhedron (Zr-AZDA) has been prepared and applied in CO₂ fixation with epoxides. The inherent azo groups not only endow Zr-AZDA with good solubilization, but also act as basic sites to enrich CO₂ showing efficient synergistic catalysis as confirmed by TPD-CO₂ analysis. XPS results demonstrate that the Zr active sites in Zr-AZDA possess suitable Lewis acidity, which satisfies both substrates activation and products desorption. DFT calculation indicates the energy barrier of the rate-determining step in CO₂ cycloaddition could be reduced remarkably (by ca. 60.9 %) in the presence of Zr-AZDA, which may rationalize the mild and efficient reaction condition employed (80 °C and 1 atm of CO₂). The work provides an effective multi-functional cooperative method for improvement of CO₂ cycloaddition.

Full text of DOI:10.1002/chem.202102089

Full Paper
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Chemistry—A European Journal
Azo-Functionalized Zirconium-Based MetalÀ Organic
Polyhedron as an Efficient Catalyst for CO2 Fixation with
Epoxides
Jia Tang,[a] Fen Wei,[a] Shujiang Ding,[b] Xiaoxia Wang,*[a] Guanqun Xie,*[a] and Hongbo Fan[a]
Abstract: Chemical fixation of CO2 as C1 source at ambient
temperature and low pressure is an energy-saving way to
make use of the green-house gas, but it still remains a
challenge since efficient catalyst with high catalytic active
sites is required. Here, a novel monoclinic azo-functionalized
Zr-based metalÀ organic polyhedron (Zr-AZDA) has been
prepared and applied in CO2 fixation with epoxides. The
inherent azo groups not only endow Zr-AZDA with good
solubilization, but also act as basic sites to enrich CO2
showing efficient synergistic catalysis as confirmed by TPD-
CO2 analysis. XPS results demonstrate that the Zr active sites
in Zr-AZDA possess suitable Lewis acidity, which satisfies both
substrates activation and products desorption. DFT calcula-
tion indicates the energy barrier of the rate-determining step
in CO2 cycloaddition could be reduced remarkably (by ca.
60.9%) in the presence of Zr-AZDA, which may rationalize the
°
mild and efficient reaction condition employed (80 C and
1 atm of CO2). The work provides an effective multi-functional
cooperative method for improvement of CO2 cycloaddition.
Introduction
both homogenous catalysts (such as halogen bond donor
catalysts,[9] phosphonium salts,[10,11] ionic liquids,[12–14] alkali metal
halides[15,16] and metal-salen complexes[17–22]), and heterogene-
ous catalysts (such as functional zeolite,[23] organosilica,[24]
porous ionic polymers,[25] ionic liquid-supported solids,[26,27]
metal oxides,[28–30] polymers,[31] organic network,[32,33] porous
carbons,[34–36] bifunctional catalysts[37,38] and metalÀ organic
frameworks (MOFs)) have been available for the CO2 fixation
into cyclic carbonates.[39,40] Among these, technically recoverable
heterogeneous catalysts such as MOFs have attracted much
attention due to their characteristics of high surface area,
uniform pore structure and tunable functionalities.[42–50] How-
ever, relatively low catalytic activity and harsh reaction con-
ditions (elevated temperature and high pressure) are frequently
encountered, which urges researchers to develop novel types of
catalysts possessing high activity under milder reaction con-
ditions.
The increase of global CO2 concentration, which mainly
originates from the burning of fossil fuels, has caused serious
environmental problems such as global warming, climate
changes and sea-level rise, and aroused the recognition that
global CO2 concentration must be reduced.[1,2] The efficient
transformation of CO2 into high value chemicals is one of the
most important solutions and has attracted much interest, since
CO2 is also an economical and reliable C1 feedstock and can be
used to synthesize numerous fine chemicals by CÀ C, CÀ O, and
CÀ N bonds formation.[3] Among various reactions that incorpo-
rate CO2,[4,5] the cycloaddition with epoxides for the synthesis of
cyclic carbonates continues to be a hot subject partly because
of the significance of the product cyclic carbonates, which have
been widely used as aprotic polar solvents, monomers, chemical
intermediates and electrolytes.[6–8]
However, owing to the high thermodynamic and kinetic
stability of CO2, its transformation into cyclic carbonate is
difficult and requires high energy consumption. To facilitate the
process, a number of catalysts have been developed. So far,
MetalÀ organic polyhedra (MOPs, also named as nanocages)
are a kind of metalÀ organic hybrid materials prepared by the
coordination of metal cations with organic ligands,[51] while they
are structurally more discrete compared with MOFs. Owing to
their porous structure and the relevance to a variety of
potential applications (such as adsorption, drug delivery and
catalysis), MOPs have also drawn great attention in recent
years.[52,53] It is well known that the coordinately unsaturated
metal ions of MOPs can behave as potential Lewis-acid active
sites.[54] However, the application of MOPs as Lewis-acid
catalysts for CO2 cycloaddition reaction is rare.[22,55] Recently, a
few MOPs that are highly soluble in organic solvents were
reported by functionalization with dodecoxyl groups on the
framework.[56] The soluble MOPs as homogeneous catalysts
would not have the aggregation problem, but the recycling of
the MOPs has not been well addressed. On the other hand, the
electronic properties of ligands also have an important
[a] Dr. J. Tang, F. Wei, Prof. Dr. X. Wang, Dr. G. Xie, Prof. Dr. H. Fan
Department School of Environment and Civil Engineering
Dongguan University of Technology
Dongguan 523808 (P. R. China)
E-mail: wangxx@dgut.edu.cn
gqxie@dgut.edu.cn
[b] Prof. Dr. S. Ding
Department of Applied Chemistry
School of Science
Xi’an Jiaotong University
Xi’an 710049 (P. R. China)
Supporting information for this article is available on the WWW under
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influence on the reactivity of transition metal complexes.[57]
previously.[58–60] the structure of the as prepared Zr-AZDA cage
Illuminated by the above reports and our experience in
heterogeneous catalysis, we envision it may be a good strategy
to develop structurally more versatile multifunctional MOPs
endowed with distinctive performance of homogeneous catal-
ysis and heterogeneous recovery.
In this work, we focus on the development of a novel
monoclinic Zr-based MOP (Zr-AZDA) as an efficient catalyst for
CO2 fixation, where Zr ions act as the active Lewis acid sites and
azo groups work as the effective basic sites together with the
ability to improve the solubility of the MOP. Zr sites in the
monoclinic MOP have appropriate Lewis acidity, which can not
only activate the substrates, but also benefit the desorption of
product due probably to the inductive effect of Cp ligand in Zr-
AZDA. Moreover, the catalyst possesses good recycling perform-
ance even after 8 recycles.
is very distinctive. The Zr⋯Cp distance is 2.2598(6) Å in the
trinuclear Zr clusters, and the Zr⋯O distances between Zr and
the respective μ3-O, μ2-O(H) and the carboxylic O atom of the
organic ligand are 2.071(2), 2.125(2) and 2.213(2) Å in the SBU.
Each polyhedron is connected with two adjacent polyhedra via
four OÀ H···Cl···HÀ O hydrogen bonds and one OÀ H···Cl···HÀ Cp
hydrogen bond (Figure 1d, S1).
The phase purity of Zr-AZDA was measured by powder X-
ray diffraction (PXRD) (Figure 2a). It shows the diffraction
pattern of the as-synthesized sample is consistent with the
simulated one. Thermogravimetric analysis (TGA) of the fresh
Zr-AZDA illustrates a continuous weight loss (about 20 wt%) of
°
solvent molecules at the initial stage (rt to 300 C) followed by
°
the weight loss of 27 wt% between 400~600 C indicating the
decomposition of the framework, which is similar to Zr-MOFs
(Figure 2b).[61] The TGA results indicate the good thermostability
of Zr-AZDA. The N2 adsorption of Zr-AZDA MOP shows weak
Results and Discussion
isothermal adsorption/desorption hysteresis loop, gives a
specific surface area of 4.2 m2 gÀ 1, indicating the presence of
low porosity in Zr-AZDA (Figure 2c). The composition of the
synthesized Zr-AZDA was measured by energy dispersive
spectroscopy (EDS). Several peaks corresponding to C, O, N, Cl,
and Zr are exhibited in Figure 2d. The element distribution on
the external surface tested by elemental mappings verifies the
highly uniform distribution of C, O, N, Cl, and Zr in Zr-AZDA
(Figure 2e–j). The quantitative measurement with ICP-MS shows
that Zr-AZDA contains 22 wt% Zr, which is close to the EDS
result (25.6 wt%) (Table S1) and the calculated value (25.2 wt%)
based on the crystal structure of Zr-AZDA.
The reaction of Cp2ZrCl2 (Cp=h5-C5H5) and azobenzene-4,4’-
dicarboxylic acid (AZDA) in a mixed solvent of DMF and H2O at
°
40 C for 4 days gives the Zr-AZDA MOP complex {[Cp3Zr3μ3-
O(μ2-OH)3]2(AZDA)3}·Cl2 ·3DMF in brick red crystals (Figure 1a).
Single crystal X-ray diffraction (SCXRD) shows the MOP
crystallizes in monoclinic space group P121/c1 (see CIF, CCDC
2010611). In this solvothermal reaction, Cp2ZrCl2 is hydrolyzed
to produce the trinuclear Zr clusters [Cp3Zr3μ3-O(μ2-
OH)3)(RCO2)3] (Figure 1b), which then undergoes coordination
with the AZDA ligand thus forming the Zr-AZDA cage (Fig-
ure 1c). Although several Zr-MOPs have been reported
Figure 1. (a) The photograph of the Zr-AZDA crystal sample. (b) Scheme for the formation of trinuclear zirconocene nodes. (c) The preparation of Zr-AZDA by
the coordination of the trinuclear zirconocene node with organic linkers. (d) The 1D connection of Zr-AZDA via hydrogen bonding between the trinuclear
zirconocene nodes and Cl ions (The analysis is based on single crystal data, see CIF, CCDC 2010611).
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Figure 2. (a) Powder XRD, (b) TGA profiles, (c) N2 sorption isotherms, (d) EDS, (e) SEM and (f-j) elemental (C, O, N, Cl and Zr) mapping images of the Zr-AZDA
sample.
X-ray photoelectron spectroscopy (XPS) measurements were
performed to analyze the chemical state of the Zr-AZDA MOP.
As shown in Figure 3, the presence of O, N, C, and Zr in Zr-
AZDA catalyst is confirmed by the survey spectrum (Figure 3a).
The two peaks at 288.7 and 284.8 eV in the C1s spectrogram
are attributed to CÀ O and CÀ C, respectively (Figure 3b). The
O1s peak at 531.8 eV confirms the presence of CÀ OÀ Zr
(Figure 3c), and the N1s peak at 400.2 eV is assigned to CÀ N
(Figure 3d). The deconvolution of the Zr 3d spectrum results in
two peaks for Zr 3d3/2 and Zr 3d5/2, respectively. Zr 3d5/2 binding
energy in Zr-AZDA is about 182.5 eV, which is higher than 179–
180.5 eV for Zr sub-oxides, and lower than 182.7 eV for high-
valence Zr in zirconium dioxide (Figure 3e).[61]
Investigation on the catalytic performance
Based on the structure analysis and initial examination on the
properties of the Zr-AZDA MOP, three interesting points
concerning its potential usage as a catalyst for CO2 fixation may
be noticed: 1) The accessible metal sites (Zr) can be utilized as
an acidic sites for the CO2 cycloaddition reaction, especially the
introduction of Cp might modulate the oxidation state of Zr
active sites; 2) With the help of azo group, Zr-AZDA may
dissolve better in the reaction mixture. It could be rationalized
that the OÀ H···Cl···HÀ O and OÀ H···Cl···HÀ Cp hydrogen bonds
may break down into discrete Zr-AZDA polyhedra during the
reaction leading to the monodispersed state of homogeneous
Zr-AZDA polyhedra in the reaction mixture (Figure 4a), facilitate
the contact between the Zr active sites and the substrate
molecules and improve the catalytic efficiency. Meanwhile it
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homogeneous, thus further accelerating the reaction (Fig-
ure S5).
To identify the superiority of the Zr-AZDA catalyst, compar-
ison of the effects with other homogeneous and heterogeneous
catalysts has been made. As shown in Table 1, the yield is just
27.6% with the ligand AZDA and the cocatalyst (Table 1,
entry 2). Zr-AZDA exhibits higher activity than a number of
homogeneous catalysts (such as Co(NO3)2 ·6H2O, Ni-
(CH3COOH)2 ·4H2O, Zn(CH3COOH)2 ·2H2O and Cu(NO3)2 ·3H2O)
(Table 1, entries 3–6), which indicates that Zr as an active site is
advantageous to CO2 cycloaddition. Assisted by the cocatalyst,
the catalysis with 1 mol% of Zr-AZDA could transform styrene
°
oxide into the desired carbonate in 92.6% yield at 80 C
(Table 1, entries 9). Moreover, the catalytic activity of Zr-AZDA is
close to or even higher than those of homogeneous Zr catalysts
(such as ZrCl4, Cp2ZrCl2) (Table 1, entries 7, 8). Heterogeneous
catalysts such as the well-known MOF catalysts (Zr-UiO-66, Zr-
UiO-66-NH2, Zr-UiO-67, Cu-BDC, Cu-BTC and Cd-TCPP) also
show lower activity than Zr-AZDA (Table 1, entries 10–15). By
comparison with either of the homogeneous catalysts (Table 1,
entries 3–8) or heterogeneous catalysts (Table 1, entries 10–15),
Zr-AZDA catalyst shows its superiority.
In addition, the comparison of catalytic performance of Zr-
AZDA with several catalysts reported in literatures was also
made (Table S3). The catalytic performance of Zr-AZDA is
comparable to that of the best homogeneous catalyst (such as
NaI·2H2O and Co-Salen),[15,18] and Zr-AZDA also shows higher
catalytic activity than most of the reported heterogeneous
catalysts (such as Basic-Nano-ZSM-5-Pr-MIM-OH, PIPÀ Bn-Cl, PS-
ImHI, MgÀ Al mixed Oxide, MgO nanoparticle, IT-POP-1, CNT-
ammonium salt, MOPÀ Zn-1, CuTrp MOF, Helicate cage, Nu-1000
and Hf-NU-1000).[24,25,27–29,31,36,44,55,64,65] Although the catalytic re-
action conditions used in the literatures are versatile, in general
Figure 3. XPS patterns of (a) the survey, (b) C 1s signals, (c) O 1s signals, (d)
N 1s signals and (e) Zr 3d signals of the Zr-AZDA catalyst.
would precipitate out by adding poor specific solvent, thereby
making homogeneous catalysis and heterogeneous recovery of
Zr-AZDA feasible (Figure 4b); 3) The azo group in the ligand
could enhance CO2 enrichment by the solitary electron
pairs.[62,63]
Bearing the consideration in mind, it was then applied in a
model reaction of CO2 cycloaddition with styrene oxide (Fig-
ure S2). The solvents were firstly screened. As shown in
Table S2, compared with the solvents of ethanol, dioxane,
CH3CN, toluene, xylene and DMF, solvent-free condition can
afford much higher yield (86%) (Table S2, entry 7). The study
shows both Zr-AZDA and cocatalyst are indispensable for good
catalytic activities. As shown in Figure S3, the use of Zr-AZDA
alone did not afford the desired product while the catalysis
with TBAB co-catalyst alone afforded 25.1% yield. Interestingly,
the combination of Zr-AZDA and 1 mol% of n-Bu4NBr afforded
87% yield of the cyclic carbonate. By further increase of the co-
catalyst loading to 2 mol% or 4 mol%, the yields further
increase to 94% and 95%, respectively. NBu4NF and nBu4NCl as
the co-catalyst, however, are dramatically inferior to nBu4NBr
and nBu4NI (Figure S4). As for the reaction temperature (Fig-
Table 1. Cycloaddition of styrene oxide with CO2 catalyzed by various
catalysts.[a]
[c]
Entry
Catalyst
Yield [%][b]
Selectivity [%][b]
TOF[hÀ 1
]
1
2
3
4
5
6
7
8
–
25.1
27.6
60.1
34.2
79.1
40.1
82
90.5
92.6
84.2
58
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
1.05
1.15
2.51
1.43
3.3
1.67
3.42
3.77
3.86
3.51
2.42
3.64
2.04
2.53
2.07
AZDA
Co(NO3)2 ·6H2O
Ni(CH3COO)2 ·4H2O
Zn(CH3COO)2 ·2H2O
Cu(NO3)2 ·3H2O
ZrCl4
Cp2ZrCl2
Zr-AZDA
Zr-UiO-66
Zr-UiO-66-NH2
Zr-UiO-67
Cu-BDC
Cu-BTC
Cd-TCPP
9
10
11
12
13
14
15
°
ure 4c), only 9.8% yield of styrene oxide is obtained at 30 C in
24 h, while the yield rises steadily from 9.8% to 97.2% with the
87.3
49
60.7
49.7
°
°
increase of temperature from 30 C to 110 C. Especially, when
°
the reaction proceeds at 110 C, the yield reaches 71.9% in 1 h
[a] Styrene oxide (0.57 mL, 5 mmol), catalyst (0.05 mmol, 1 mol% metal
sites), CO2 (CO2 balloon, 1 atm) and nBu4NBr (0.032 g, 0.1 mmol, 2 mol%)
and 93% in 4 h. The increase of the temperature not only
facilitates the reaction dynamically, but also improves the
solubility of the Zr-AZDA catalyst and makes the reaction
°
were added to a Schlenk tube. The mixture was kept stirring at 80 C for
24 h. [b] Yield and selectivity were determined by GC-MS. [c] Mole of cyclic
carbonate produced per mole of Zr per hour.
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Figure 4. (a) Scheme for the fracture of the hydrogen bonds between Zr-AZDA polyhedra during the catalytic reaction. (b) Scheme for the strategy of
homogeneous catalysis and heterogeneous recovery of the Zr-AZDA catalyst. (c) Effect of temperature on the CO2 cycloaddition catalyzed by Zr-AZDA.
Reaction conditions: styrene oxide (0.57 mL, 5 mmol), CO2 (CO2 balloon, 1 atm), catalyst (1 mol% Zr), and nBu4NBr (0.032 g 0.1 mmol, 2 mol%) were added to a
Schlenk tube. The mixture was kept stirring at the specific temperature for 24 h. Yields were determined by GC-MS.
the Zr-AZDA catalyst has obvious advantages such as mild
reaction conditions, solvent-free at atmospheric pressure
(1 atm) and higher catalytic activity.
To expand the application scope of Zr-AZDA catalyst for the
CO2 fixation into cyclic carbonates, a variety of epoxides as
substrates were examined. As shown in Table 2, with 1 mol% of
the Zr-AZDA catalyst, propylene oxide smoothly converts into
propylene carbonate in a yield of 99.9% at ambient temper-
activity as compared with literature,[66–70] and the cyclohexene
carbonate is obtained in 15.2% yield (Table 2, entry 11).
The recovery of the catalyst is then performed. As described
in the previous experiments, Zr-AZDA catalyst can dissolve in
the reaction mixture better with the increase of the temper-
ature, thus being able to form a homogeneous reaction system.
When the temperature decreases, Zr-AZDA can precipitate from
the mixture and more complete recovery of the catalyst could
be achieved by introducing poor solvent. It was found that
ethanol with less toxicity could precipitate Zr-AZDA catalyst
well. Therefore, ethanol was used as precipitant and recovery of
the catalyst was achieved by further cooling the mixture with
ice bath followed by filtration. The recovered catalyst was
repeatedly used for catalyzing the CO2 cycloaddition with
epichlorohydrin at lower than full substrate conversion (Fig-
ure 5a–c). It is satisfying that Zr-AZDA catalyst can be recycled
for 8 times without obvious decrease of catalytic activity.
Indeed, a constant yield of ~70% with selectivity of >99.9% is
accomplished for 8 successive runs (Figure 5d). Leaching test
showed the solution almost stopped to afford more products
by removing of the Zr-AZDA catalyst and the results exhibit the
heterogeneous catalysis feature of the catalytic system (Fig-
ure S12).
°
ature of 30 C (Table 2, entry 1). With the growth of the carbon
chain length in the epoxides, the CO2 cycloaddition becomes
less efficient, as exemplified by the yields of propylene and
°
butylene carbonate (both 99.9% yield at 60 C) and decenyl
°
carbonate (78.5% yield at 80 C) (Table 2, entries 2 and 3).
Epichlorohydrin and epibromohydrin also afford excellent yields
°
of the corresponding carbonates at 80 C (Table 2, entries 4, 5;
Figure S5, S6). Butyl gylcidyl ether and tert-butyl glycidyl ether
afford the desired products in excellent yields (96.3% and
90.9% yield, respectively) (Table 2, entries 6 and 7, Figure S7).
Glycidyl phenyl ether can be transformed into the correspond-
ing product in a yield of 91.1% (Table 2, entry 8; Figure S8).
Styrene oxide is converted into styrene carbonate in 92.6%
yield (Table 2, entry 9; Figure S9). Despite the challenge posed
by the steric hindrance of benzene rings, the cycloaddition of 2-
((trityloxy)methyl)oxirane with CO2 still affords the carbonate in
20.6% yield (Table 2, entry 10). Structurally more hindered
disubstituted epoxide such as cyclohexene oxide shows lower
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Table 2. Cycloaddition of CO2 with various epoxides catalyzed by the Zr-
AZDA catalyst.[a]
Entry Substrate
Product
Yield Selectivity TOF
[f]
[%][e]
[%][e]
[hÀ 1
]
1[b]
2[b]
99.9
99.9
>99
>99
4.17
4.17
3
78.5
98.5
3.32
4[c]
5
99.9
99.2
>99
>99
4.17
4.14
6
96.3
>99
4.02
7
8
9
90.9
91.1
92.6
98.2
97
3.86
3.92
3.86
>99
10[d]
20.6
15.2
73.6
82
1.17
0.77
Figure 5. The photographs of (a) the mixture before reaction, (b) the
homogeneous reaction mixture and (c) the recovered catalyst after reaction.
(d) Recycling of the Zr-AZDA catalyst. Reaction conditions: epichlorohydrin
(5 mmol), Zr-AZDA catalyst (1 mol% Zr), CO2 (CO2 balloon, 1 atm) and
nBu4NBr (0.032 g, 2 mol%) were added to a Schlenk tube. The mixture was
11
[a] Epoxide substrate (5 mmol), Zr-AZDA catalyst (1 mol% Zr), CO2 (CO2
balloon, 1 atm) and nBu4NBr (0.032 g, 0.1 mmol, 2 mol%) were added to a
°
kept stirring at 80 C for 3 h. Yield and selectivity were determined by GC-
MS.
°
°
Schlenk tube. The mixture was kept stirring at 80 C for 24 h; [b] 30 C; [c]
°
°
60 C; [d] 110 C. [e] Yield and selectivity were determined by GC-MS. [f]
Moles of cyclic carbonates produced per mole of Zr per hour.
Compared with Zr-UiO-66, which exhibits two desorption peaks
°
°
at 173 C and 336.5 C, Zr-AZDA presents two desorption peaks
°
°
Investigation on the reaction mechanism
at 162.1 C and 321 C. Besides the slight shift of the peaks to
lower temperatures, the peak intensity of Zr-AZDA is higher
than that of Zr-UiO-66, which indicates that Zr-AZDA with an
azo group has a stronger basicity. So it could be rationalized
that CO2 molecules are enriched in the framework of Zr-AZDA
due to the presence of the azo group as Lewis basic site. Thus
Zr-AZDA as a catalyst combines both of the Lewis acid Zr
centers and Lewis base azo group.[62]
To better understand the catalytic activity of Zr sites in the
monoclinic Zr-AZDA MOP, comparison of Zr-UiO-66 MOF was
further made. As shown in Figure 6a, Zr-AZDA catalyst exhibit
obviously higher catalytic activity than Zr-UiO-66. Determina-
tion of XPS of Zr-AZDA and Zr-UiO-66 shows the binding energy
of Zr 3d3/2 and 3d5/2 in Zr-AZDA are 184.73 eV and 182.28 eV,
respectively, which are lower than 185.03 eV and 182.63 eV for
Zr 3d3/2 and 3d5/2 in Zr-UiO-66 (Figure 6b, S13). The Cp ligand in
the trinuclear Zr clusters may have significant influence on the
Zr active sites by the inductive effect, thus the Zr shows lower
oxidation state in Zr-AZDA than that in Zr-UiO-66. It is rational
to propose the suitable oxidation state of Zr sites is an
important factor for the high catalytic activity, which is
beneficial not only to the activation of the substrate, but also to
the desorption of product.[71–73]
According to the previous reports and our experimental
results, a plausible mechanism for the CO2 cycloaddition
reaction under the catalysis of Zr-AZDA is proposed.[77,78] As
shown in Figure 7a, the Lewis acidic Zr sites in the trinuclear Zr
clusters of Zr-AZDA are inclined to coordinate with the epoxide
substrate, which could activate the epoxy ring, weaken the
associated CÀ O bonds and facilitate the attack of BrÀ of the co-
catalyst on the electron more positive carbon of the epoxy ring.
Meanwhile, the CO2 molecules are readily attacked by the
oxygen anion of the ring-opening intermediates to generate a
metal-carbonate intermediate. Finally, the intramolecular nucle-
ophilic substitution of the bromo with the carbonate anion
finishes the ring closure and gives the cyclic carbonate.
On the other hand, it is reported azo group as Lewis basic
site could enhance CO2 enrichment by the solitary electron
pairs.[60,74–76] To verify the CO2 enrichment of azo group in the
framework of Zr-AZDA, the basicity of the azo group in Zr-
AZDA catalyst was characterized by CO2-TPD (Figure 6c).
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reaction barrier decreases significantly in the presence of the
catalyst as shown in Figure 7c. Firstly, Zr-AZDA catalyst absorbs
and activates epichlorohydrin with its acidic Zr site, and the
process is exothermic with an energy barrier value of À 0.96 eV.
Then BrÀ approaches the electron more positive carbon (the
more substituted carbon of epichlorohydrin) with an energy
barrier value of À 0.18 eV. The Zr-AZDA would also absorb CO2
with its basic center (the azo group) and activate CO2, which is
spontaneous since the energy barrier value is À 0.83 eV. The
following attack of BrÀ on epichlorohydrin to undergo the ring-
opening reaction proceeds with a DG value of 1.38 eV, which
indicates an endothermic and also a rate-determining step. By
comparison with the energy barrier value of 2.22 eV in blank
experiment, the energy barrier is reduced by 60.9%, thus
explains why the reaction has been accelerated remarkably.
Subsequent intermolecular insertion of CO2 and intramolecular
substitution proceed readily to afford the desired product with
a DG value of À 0.97 eV. Finally, departure of BrÀ and desorption
of the product from Zr-AZDA catalyst are slightly endothermic
with the respective DG values of 0.13 eV and 0.47 eV. It is
noteworthy that the Lewis acidity of the Zr is strong enough to
activate the epoxide but not too strong to prohibit the
desorption of products according to the DFT calculation, thus
contributes well to the catalytic efficiency.[56,64]
Zr-AZDA catalyst still has excellent catalytic activity after
repeated recycling, which should be related to its robust
structure. Firstly, as shown in Figure 2b, Zr-AZDA catalyst has
good thermal stability and no decomposition occurs below
°
400 C. Secondly, the FTIR spectrum of the recovered mono-
dispersed Zr-AZDA polyhedra is found to be identical with that
of the fresh Zr-AZDA (Figure 8a). Finally, XPS was used to
characterize the chemical environment changes of Zr-AZDA
before and after the reaction. The recovered Zr-AZDA polyhedra
show almost the same Zr 3d signal with the fresh one
(Figure 8b, S14), which indicates the coordination bonds
between Zr ions and organic ligands are robust enough,[79] and
remain intact during the catalytic process. Therefore, the high
stability of the main structure of Zr-AZDA catalyst guarantees
its high activity after several cycles.
Conclusion
Figure 6. (a) Cycloaddition of styrene oxide with CO2 catalyzed by Zr-AZDA
(red) and Zr-UiO-66 (blue) catalysts. Reaction conditions: styrene oxide
(0.57 mL, 5 mmol), CO2 (1 atm), catalyst (1 mol% Zr), and nBu4NBr (0.032 g
0.1 mmol, 2 mol%) were added to a Schlenk tube. The mixture was kept
In this paper, a novel monoclinic Zr-based MOP (Zr-AZDA) has
been synthesized and applied in the fixation of CO2 into cyclic
carbonates. With the assistance of the inherent azo group, Zr-
AZDA catalyst can dissolve readily in the reaction mixture at
moderate temperature, thus forming a homogeneous catalytic
system. Furthermore, TPD-CO2 test showed that azo groups as
basic sites can induce CO2 enrichment. In addition, the azo
groups are located closely around the Zr active sites facilitating
an efficient synergistic catalysis. XPS results showed the Zr
active sites possessed lower oxidation state thereby leading to
a lower Lewis acidity as compared with common Zr-MOF (such
as Zr-UiO-66). DFT results suggested that the desorption of the
product was slightly endothermic, which indicated lower
energy barrier for the product desorption. Therefore, the Lewis
°
stirring at 80 C for 12 h. Yield and selectivity were determined by GC-MS. (b)
XPS spectra of Zr 3d signals of Zr-AZDA and Zr-UiO-66. (c) TPD-CO2 profiles
of Zr-AZDA and Zr-UiO-66.
To study the activation energy stepwise for the CO2 cyclo-
addition on Zr-AZDA, theoretical calculations were performed
using the plane-wave DFT code from the Vienna ab initio
simulation package (VASP). The reaction between CO2 and
epichlorohydrin in the absence of any catalyst is endothermic
with an energy barrier value of 2.22 eV (Figure 7b), which
indicates high energy is required to overcome the energy
barrier for the synthesis of cyclic carbonate. In contrast, the
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Figure 7. (a) A plausible mechanism for the CO2 cycloaddition reaction over Bu4NBr/Zr-AZDA catalyst. Relative Gibbs energy of each step for CO2 cycloaddition
with epichlorohydrin (b) without catalyst and (c) catalyzed by Zr-AZDA catalyst. Color scheme: Pink for Br, sea green for Zr, red for O, gray for C, and bright
green for Cl.
Figure 8. (a) FTIR spectra and (b) XPS spectra of Zr 3d signal of fresh Zr-AZDA and the recovered Zr-AZDA sample.
acidity of Zr active sites is well modulated so that it could not
only effectively activate the substrate, but still be beneficial to
the product desorption. DFT calculation also confirmed Zr-
AZDA catalyst could significantly reduce the energy barrier of
the rate-determining step (by �60.9%), which rationalized the
relatively mild conditions for the CO2 cycloaddition reaction.
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Notably, the catalyst could be recovered readily, and recycling
tests showed it still exhibited high activity for the cycloaddition
of CO2 after 8 recycles. This work is a useful trial in developing
multifunctional and readily recoverable MOP catalysts for CO2
fixation under mild conditions.
196100041051), National Natural Science Foundation of China
(No. 51773165 and 51973171) and Young Talent Support Plan
of Xi’an Jiaotong University.
Conflict of Interest
Experimental Section
The authors declare no conflict of interest.
Preparation of azobenzene-4,4’-dicarboxylic acid (AZDA): The ligand
AZDA was synthesized according to the literature.[56] Five grams
(30 mmol) of 4-nitrobenzoic acid and 17 g of sodium hydroxide
Keywords: CO2 conversion · CO2 fixation · Cyclic carbonates ·
MOP · Zr-based metalÀ organic polyhedron
°
were added in 75 mL of water and heated at 50 C until the solid
was dissolved, and then the solution was cooled to room temper-
ature and kept stirring followed by dropwise addition of 50 mL of
°
hot glucose solution (80 C, containing 30 g of glucose). The
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Manuscript received: June 12, 2021
Accepted manuscript online: July 19, 2021
Version of record online: ■■■, ■■■■
Chem. Eur. J. 2021, 27, 1–11
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FULL PAPER
Novel zirconium-based metal organic
polyhedron (MOP) with azo function-
ality has been synthesized and
Dr. J. Tang, F. Wei, Prof. Dr. S. Ding,
Prof. Dr. X. Wang*, Dr. G. Xie*,
Prof. Dr. H. Fan
exhibits high catalytic activity for CO2
fixation. The azo group could enhance
the solubility of the MOP as well as
being good Lewis base sites to enrich
CO2. DFT calculation showed Zr-AZDA
MOP with the Lewis activity of Zr sites
could dramatically reduce the energy
barrier of CO2 cycloaddition.
1 – 11
Azo-Functionalized Zirconium-Based
MetalÀ Organic Polyhedron as an
Efficient Catalyst for CO2 Fixation
with Epoxides