A lanthanide MOF catalyst with an excellent thermal stability for the synthesis of polycarbonate diol

Article abstract of DOI:10.1007/s13738-020-01929-8

A lanthanide metal–organic framework (MOF) Yb(BTC)(H₂O)·(DMF) (BTC = 1,3,5-benzenetricarboxylate, DMF = N,N-dimethylformamide) was prepared from Yb(NO₃)₃·5H₂O and H₃BTC using the solvothermal method and characterized by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, N₂ adsorption–desorption technology, and temperature-programmed desorption. The results show that Yb(BTC)(H₂O)·(DMF) is a lanthanide MOF with an excellent thermal stability. The coordinated water molecules and DMF guest molecules were removed from the channels by calcination to form a porous solid Yb (BTC) with coordinatively unsaturated Yb³⁺ sites. The BET surface area and pore size of Yb(BTC) are 695?m²/g and 0.68?nm, respectively. As a heterogeneous Lewis acid catalyst, the activity of Yb(BTC) for the synthesis of polycarbonate diol (PCDL) via transesterification between diphenyl carbonate and diol was investigated. Compared with nine reported catalysts, Yb(BTC) exhibits the highest catalytic activity for the synthesis of PCDL.

Full text of DOI:10.1007/s13738-020-01929-8

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-01929-8
ORIGINAL PAPER
A lanthanide MOF catalyst with an excellent thermal stability
for the synthesis of polycarbonate diol
Liping Wang1 · Jiao He1 · Xueying Chen1 · Yanling Lv1
Received: 5 June 2019 / Accepted: 30 March 2020
© Iranian Chemical Society 2020
Abstract
A lanthanide metal–organic framework (MOF) Yb(BTC)(H2O)·(DMF) (BTC=1,3,5-benzenetricarboxylate, DMF=N,N-
dimethylformamide) was prepared from Yb(NO3)3·5H2O and H3BTC using the solvothermal method and characterized by
Fourier transform infrared spectroscopy, X-ray difraction, thermogravimetric analysis, scanning electron microscopy, N2
adsorption–desorption technology, and temperature-programmed desorption. The results show that Yb(BTC)(H2O)·(DMF)
is a lanthanide MOF with an excellent thermal stability. The coordinated water molecules and DMF guest molecules were
removed from the channels by calcination to form a porous solid Yb (BTC) with coordinatively unsaturated Yb3+ sites. The
BET surface area and pore size of Yb(BTC) are 695 m2/g and 0.68 nm, respectively. As a heterogeneous Lewis acid catalyst,
the activity of Yb(BTC) for the synthesis of polycarbonate diol (PCDL) via transesterifcation between diphenyl carbonate
and diol was investigated. Compared with nine reported catalysts, Yb(BTC) exhibits the highest catalytic activity for the
synthesis of PCDL.
Keywords Lanthanide metal–organic framework · Catalyst · Polycarbonate diol · Transesterifcation
Introduction
aliphatic alkylidene and carbonate groups in the main chain.
Because of the strong cohesion between carbonate groups,
polycarbonate-based WPU has more excellent hydrolysis
resistance, aging resistance, weathering resistance, oil resist-
ance, wear resistance, mildew resistance, and mechanical
properties than polyester-based and polyether-based WPUs
[10–12]. Besides, high biostability, good biocompatibility,
and in vivo oxidation resistance of polycarbonate-based
WPU have also been confrmed [13]. Therefore, the develop-
ment of PCDL and polycarbonate-based WPU has attracted
increasingly more attention.
Waterborne polyurethane (WPU), a polyurethane emulsion
where water is used as the dispersing medium, is a class
of environment-friendly polymer with the characteristics of
noninfammability, incombustibility, and nontoxicity [1].
WPU has been widely applied in many felds such as paint
[2], synthetic leather [3], and adhesive [4]. The performance
of WPU including thermal stability, fexibility, and physi-
cal and mechanical properties is signifcantly afected by
the structure of soft segment [5–7]. WPU made of poly-
ester polyol with strong polarity exhibits better mechani-
cal property, but the ester groups in polyester-based WPU
can be hydrolyzed. Therefore, the hydrolysis resistance of
polyester-based WPU is poor [8]. WPU produced from poly-
ether polyol has better hydrolysis resistance and fexibility.
However, because of the presence of ether groups, polyether-
based WPU has poor heat resistance and mechanical proper-
ties [6, 9]. Polycarbonate diol (PCDL) is a polyol containing
Several nonphosgene methods have been developed for
the synthesis of PCDL, including the ring-opening polym-
erization of six and seven-membered cyclic carbonates [14],
copolymerization of CO2 with epoxides [15], and transes-
terifcation between organic carbonate and diol [16–24].
Among these methods, transesterifcation is regarded as a
more promising route because of many advantages such
as less amount of catalyst, mild reaction conditions, fex-
ibility for preparing PCDLs with multiple structures, and
controlled planning for relative molecular mass of the target
products. Various Lewis acid catalysts including Ti(OBu)4,
Zn(CH3COO)2, and Yb(acac)2 are commonly used for this
transesterifcation, but the drawback of these catalysts is that
* Liping Wang
wanglp_csu@163.com
1
College of Chemistry and Environmental Science, Qujing
Normal University, Qujing 655011, China
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
they cannot be separated from the products and recycled.
Thus, it is essential to develop heterogeneous catalysts.
Metal–organic frameworks (MOFs) constructed from
metal ions or clusters and organic ligands have been known
as a class of porous crystal materials with periodic network
structures [25]. Because of their large surface areas, orderly
pore structures, adjustable pore sizes, abundant active sites,
and easily modifed frameworks, more attention has been
paid to the applications of MOFs in heterogeneous catalysis
[26, 27]. Trivalent rare earth metal ions are hard acids and
easily coordinate with oxygen-containing organic ligands.
Moreover, they have a higher coordination number than the
main group metal ions and transition metal ions. Therefore,
the resulting MOFs have better thermal stability.
Runjie Chemical Reagent Co., Ltd.), 1,4-butanediol (BDO,
98%, Tianjin Fuchen Chemical Reagent Factory), and
dichloromethane (99.5%, Chengdu Kelong Chemical Rea-
gent Factory) were used to synthesize PCDL.
Preparation and activation of catalyst
Yb(BTC)(H2O)·(DMF) was prepared according to the fol-
lowing procedure. Yb(NO3)3·5H2O (0.1761 g, 0.39 mmol)
and H3BTC (0.0824 g, 0.39 mmol) were dissolved in a sol-
vent containing DMF (40 mL), cyclohexanol (8 mL), and
H2O (8 mL). Two drops of n-butylamine were added to the
solution, forming a precipitate. Then, a few drops of 2 mol/L
HNO3 were added to the mixture, and the precipitate dis-
solved. The resulting solution was placed in a 100 mL vial
and kept at 90 °C for 19 h. The precipitate was collected by
fltration and washed with ethanol (3 × 10 mL) to remove
Yb(NO3)3 and H3BTC not involved in the reaction. Finally,
the solid was dried at 100 °C for 24 h.
However, studies have been rarely reported on MOFs
based on trivalent rare earth metal ions and aromatic car-
boxylic acid used to catalyze the synthesis of PCDL via
the transesterifcation between carbonate and diol. In this
study, a Yb-containing MOF Yb(BTC)(H2O)·(DMF)
(BTC=1,3,5-benzenetricarboxylate, DMF=N,N-dimethyl-
formamide) with an excellent thermal stability was prepared
and characterized, and its desolvated derivative Yb(BTC)
with coordinatively unsaturated Yb3+ sites was used as a
Lewis acid solid catalyst for the synthesis of PCDL via the
transesterifcation between diphenyl carbonate (DPC) and
aliphatic diol. To illustrate the activity of Yb(BTC), the
activities of Yb(BTC) were also compared with those of
triethylamine (TEA), Zn(CH3COO)2, Ti(OBu)4, TiO(acac)2,
KF/Al2O3, MgAl hydrotalcite (MgAl HT), Mg–Fe/Ti layered
double hydroxide (Mg–Fe/Ti LDH), Zn–Al–CO3 layered
double hydroxide (Zn–Al LDH), and MOF-5 in a refuxing
batchwise reactor. In addition, the efect of catalyst amount
on the transesterifcation reaction was evaluated. A possible
mechanism is proposed for the synthesis of PCDL via the
transesterifcation of DPC with diol catalyzed by Yb(BTC).
To thoroughly remove H2O molecules coordinated with
Yb3+ from the channels to form a porous solid Yb(BTC)
with coordinatively unsaturated Yb3+ sites, the sample was
calcined at 350 °C for 4 h and then at 200 °C for 24 h under
8×104 Pa pressure.
Catalytic reaction
The transesterifcation between organic carbonate and ali-
phatic diol to synthesize PCDL involves two processes.
During the transesterifcation, certain amounts of organic
carbonate, aliphatic diol, and catalyst were introduced into a
100 mL three-neck fask equipped with a mechanical stirrer,
thermometer, and refux condenser under nitrogen atmos-
phere. The reaction temperature was gradually increased
to 180 °C under N2 atmosphere. These conditions were
maintained for 2–3 h until the distillate was not generated.
During the polycondensation, the reaction temperature was
decreased to 160 °C, and the pressure was decreased to
9 × 103 Pa. This process was continued for 4 h. Then, the
product was naturally cooled to room temperature. Then,
it was dissolved in dichloromethane and precipitated in
methanol.
Experimental
Chemicals
Ytterbium(III) nitrate pentahydrate (Yb(NO3)3·5H2O,
99.99%, Guangdong Wengjiang Chemical Reagent Co.,
Ltd.), 1,3,5-benzenetricarboxylic acid (H3BTC, 98%, Shang-
hai Aladdin Biochemical Technology Co., Ltd.), n-butyl-
amine (98%, Shanghai Aladdin Biochemical Technology
Co., Ltd.), DMF (99.5%, Tianjin Fengchuan Chemical Rea-
gents Technology Co., Ltd.), and ethanol (99.7%, Tianjin
Fengchuan Chemical Reagents Technology Co., Ltd.) were
used to prepare Yb(BTC). DPC (99%, Shanghai Aladdin
Biochemical Technology Co., Ltd.), dimethyl carbonate
(DMC, 98%, Shandong University of Petroleum Huasheng
Chemical Co., Ltd.), 1,6-hexanediol (HDO, 98%, Shanghai
The product poly(1,4-butanediol) carbonate diol prepared
from DPC and BDO was abbreviated as PBC, and poly(1,6-
hexanediol) carbonate diol generated from DPC and HDO
was referred as PHC.
Characterization
X-ray difraction (XRD) analyses were carried out using a
Rigaku Ultima IV difractometer (Rigaku, Japan). Scanning
electron microscopy (SEM) images were recorded using a
Phenom Pro X electron microscope (Phenom, Holland).
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Journal of the Iranian Chemical Society
Fourier transform infrared (FTIR) spectroscopy was car-
ried out using a Nicolet Magna-IR 560 Fourier infrared
spectrometer (Nicolet, USA). Pore size and specifc surface
area data were obtained using a Kubo-X1000 pore size and
specifc surface area analyzer (Beijing Biaode Electronic
Technology Co., Ltd., China). Thermal stability analysis
was performed using a Mettler SDTA 851e thermogravi-
metric (TG) analyzer (Mettler, Switzerland) at a heating rate
of 10 °C/min from 50 to 800 °C in nitrogen. Temperature-
programmed desorption (TPD) curve was obtained using
a PCA-1200 Chemisorption Analyzer (Beijing Biaode
Electronic Technology Co., Ltd., China) using pyridine
(C5H5N) as a basic adsorbent. The content of carbonate in
the distillate was determined using a GC-2010 gas chro-
matograph (Shimadzu, Japan) equipped with a SUPELCO
Equity™-1 capillary column (30 m×0.25 mm×0.25 μm),
and the conversion of carbonate was calculated using For-
mula (1). The number average relative molecular mass (Mn)
and polydispersity index (PDI) of PCDL were determined
using a WATERS gel chromatography (Waters, USA) using
tetrahydrofuran as the solvent. The hydroxyl value (NOH) of
PCDL was determined using o-phthalic anhydride-pyridine
titration method, and the hydroxyl functionality of PCDL
was calculated using Formula (2). 1H NMR spectra of PCDL
were recorded using a Bruker Avance III 500 MHz spec-
trometer (Bruker, Germany).
(c)
(b)
(a)
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber /cm-1
Fig. 1 FTIR spectra of a H3BTC and Yb(BTC)(H2O)·(DMF), b
before and c after calcination
(H2O)·(DMF) indicates that the COOH groups in H3BTC
are completely deproteinized and participates in the coor-
dination with Yb3+. The bands observed at 1561 cm−1 and
1381 cm−1 can be attributed to the asymmetric stretching
vibration and symmetric stretching vibration of COO−,
respectively. Δν = 180 cm−1 < 200 cm−1 implies that the
coordination mode of H3BTC and Yb3+ is bridging biden-
tate [28]. The bands at 1200 cm−1 and 941 cm−1 refect the
in-plane bending vibration of C–H. The bands at 771 cm−1,
741 cm−1, and 713 cm−1 represent the out-of-plane bend-
ing vibration of C–H. The band at~1623 cm−1 corresponds
to the vibrational band of aromatic ring. At the same time,
it was observed that the main characteristic bands of sam-
ple after calcining at 350 °C for 4 h are consistent with
those of solid before calcination, except that the intensity
of broad band at 3500 cm−1 attributing to the vibration of
H2O decreases signifcantly. This indicates that the struc-
ture of Yb(BTC)(H2O)·(DMF) remained unchanged, and
the terminal H2O molecules could be efectively removed
by calcination.
m0 − mt
C∕% =
× 100
(1)
m0
where C is the conversion of carbonate; m0 is the initial
mass of carbonate, g; mt is the mass of carbonate in the
distillate, g.
Mn × NOH
F =
(2)
56.1 × 1000
where F is the hydroxyl functionality of PCDL, Mn is the
number-average relative molecular mass of PCDL, g/mol;
NOH is the hydroxyl value of PCDL, mg KOH/g.
TG analysis
Results and discussion
Characterization of Yb(BTC)(H2O)·(DMF)
FTIR spectra
The chemical composition of Yb(BTC)(H2O)·(DMF) was
further determined by TG analysis. The thermal decompo-
sition behavior of Yb(BTC)(H2O)·(DMF) in the range of
50–800 °C is shown in Fig. 2. The weight loss of 19.1%
between 100 and 420 °C is equal to the removal of one
water molecule and one DMF molecule per formula unit
(calculated: 19.3%). The onset of weight loss at 590 °C
that terminated at 710 °C can be attributed to the decom-
position of BTC ligand. Clearly, Yb(BTC)(H2O)·(DMF)
does not decompose until the temperature reaches 590 °C,
FTIR spectra of H3BTC and Yb(BTC)(H2O)·(DMF)
are shown in Fig. 1. Compared with H3BTC, the
absence of stretching vibration band of C=O in car-
boxyl group (1700–1680 cm−1), stretching vibration
band (3000–2500 cm−1), and bending vibration band
(950–890 cm−1) of O–H on the FTIR spectrum of Yb(BTC)
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Journal of the Iranian Chemical Society
peaks indicate that the sample has high crystallinity. The
XRD pattern in this case is consistent with those of Dy(BTC)
(H2O)·DMF and Nd(BTC)(H2O)·DMF, indicating that these
three lanthanide MOFs are isostructural [29, 30]. The main
difraction peaks are located at 8.62°, 10.58°, 12.21°, 13.66°,
17.34°, 18.36°, 19.35°, 20.32°, 22.97°, 25.38°, 26.07°,
26.86°, 27.53°, 28.20°, 32.12°, and 35.09°, corresponding
to the (100), (101), (110), (111), (200), (201), (210), (121),
(212), (221), (300), (301), (310), (131), (321), and (400)
lattice planes, respectively. Among these, the difraction
peaks of (100), (101), (201), and (121) lattice planes are
relatively stronger. In addition, Fig. 3 shows that when the
sample is calcined at 350 °C, the position of difraction peak
is unchanged, except that the intensity of some difraction
peaks slightly changes, indicating that high-temperature cal-
cination does not destroy the crystal structure of Yb(BTC)
(H2O)·(DMF). According to TG analysis and XRD patterns,
Yb(BTC)(H2O)·(DMF) has an excellent thermal stability.
The structure analysis shows that Yb(BTC)(H2O)·(DMF)
crystallizes in the tetragonal space group P4122, where
a≤10.20958 Å, c≤14.39981 Å, and V≤1500.97 Å3. Each
Yb atom is coordinated with six oxygen atoms from the
six carboxylate groups of diferent BTC ligands and one
oxygen atom from a terminal water molecule. Yb atoms
are interconnected by fve carboxy groups from four BTC
ligands to form a one-dimensional (1D) inorganic chain,
–Yb–O–C–O–Yb–, along the b axis (Fig. 4a). These 1D
chains link to each other by BTC to form a three-dimen-
sional (3D) framework in which the water molecules are
pointed to the channels (Fig. 4b).
100
80
19.1wt%
60
0
200
400
Temperture /oC
600
800
Fig. 2 Thermogravimetric curve of Yb(BTC)(H2O)·(DMF)
(e)
(d)
(c)
(b)
(a)
5
10
15
20
25
30
35
40
N2 adsorption–desorption isotherm and SEM images
The pore size, surface area, and micromorphology of
Yb(BTC) were investigated by N2 adsorption–desorption
technology and SEM images. The results are shown in
Fig. 5. As shown in Fig. 5a, when the p/p0 is slightly higher
than 0, the adsorption capacity of sample sharply increases.
When p/p0 exceeds 0.22, the adsorption capacity reaches the
saturation. The isotherm appears to be a Type I adsorption,
indicating that the sample has abundant micropore structure
and uniform pore size. The BET surface area of Yb(BTC)
and mesopore size are 695 m2/g and 0.68 nm, respectively.
Figure 5b clearly shows that Yb(BTC) is composed of rod-
like nanostructures with a length of~70 μm.
Fig. 3 XRD patterns of Yb(BTC)(H2O)·(DMF): a the simulated XRD
pattern, b the as-synthesized sample, the sample, c after calcination at
200 °C, d after calcination at 350 °C and e after calcination at 400 °C
indicating that Yb(BTC)(H2O)·(DMF) is a lanthanide MOF
with a higher thermal stability than the reported Dy(BTC)
(H2O)·DMF (400 °C) [29] and Nd(BTC)(H2O)·DMF
(525 °C) [30]. Besides, the TG analysis once again proves
that the H2O and DMF molecules are dissociated and
removed from the framework by calcination, and Yb3+ ions
become coordinatively unsaturated in the resulting porous
solid Yb(BTC).
Comparison of catalytic activity
XRD patterns
The synthesis of PCDL via the transesterifcation between
diol and carbonate can be divided into two processes: trans-
esterifcation and polycondensation. The activity of catalyst
in transesterifcation was evaluated by the conversion of
carbonate (C). The activity of catalyst in polycondensation
The XRD patterns of Yb(BTC)(H2O)·(DMF) were analyzed
and refned using Rietveld method and EXPGUI-GSAS soft-
ware, and the simulated and measured XRD patterns are
shown in Fig. 3. The strong, sharp, and narrow difraction
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Journal of the Iranian Chemical Society
Fig. 4 a The one-dimensional
inorganic chain viewed along
the b axis and b the three-
dimensional framework viewed
along the c axis
250
(b)
(a)
200
150
100
50
0.09
0.06
0.03
0.00
0
5
10 15 20 25
0
Pore diameter/nm
0.0
0.2
0.4
0.6
p/p
0.8
1.0
1.2
0
Fig. 5 a N2 adsorption and desorption isotherm and pore size distribution and b SEM image of Yb(BTC)
was determined from the Mn and NOH of PCDL. TEA,
Zn(CH3COO)2, Ti(OBu)4, TiO(acac)2, KF/Al2O3, MgAl
HT, Mg–Fe/Ti LDH, Zn–Al LDH, and MOF-5 are efcient
catalysts for the synthesis of PCDL via the transesterifcation
between carbonate and diol [20–24]. The efect of diferent
catalysts on the synthesis of PCDL via the transesterifcation
is shown in Table 1.
value was prepared. When titanium-containing compounds
were used as the catalysts, the obtained PCDL becomes
yellow during storage, afecting the subsequent application
of PCDL. Besides, Ti(OBu)4 is unstable in air. KF/Al2O3,
MgAl HT, Mg–Fe/Ti LDH, Zn–Al LDH, and MOF-5 are
heterogeneous catalysts and can be separated from the prod-
uct. However, KF/Al2O3 is a supported catalyst, and the
loss of active component KF decreases the activity. When
Mg–Fe/Ti LDH was used as the catalyst in the reaction, a
higher conversion of DPC was obtained in transesterifca-
tion. However, the amount of catalyst used in the reaction is
more, and the reaction time is the longest. At the same time,
the hydroxyl functionality of the obtained product is the far-
thest from the theoretical value of 2. TEA, KF/Al2O3, MgAl
HT, Mg–Fe/Ti LDH, and Zn–Al LDH are basic catalysts.
A basic catalyst can easily initiate the decarboxylation of
carbonate, thus increasing the ether group content in PCDL
Among these catalysts, TEA, Zn(CH3COO)2, Ti(OBu)4,
and TiO(acac)2 are homogeneous catalysts and cannot be
separated from the product. As shown in Table 1, the amount
of TEA used in the reaction is much, and the remaining
TEA in the PCDL has a negative efect on the synthesis of
WPU. When Zn(CH3COO)2 was used as a catalyst, the high-
est conversion was obtained in transesterifcation. However,
the catalyst amount used in the reaction is the most, the reac-
tion time is also longer, and PCDL with the lowest number-
average relative molecular mass and the highest hydroxyl
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Journal of the Iranian Chemical Society
Table 1 Comparison of activities between Yb(BTC) and catalysts reported
Catalyst
θa/°C (tb/h)
n(diol):n(carbonate) Wc/% C/ %
Mn/g/mol NOH/mg KOH/g
F
References
TEA
100 (2), 160–180 (4)
150–154 (9.5)
120–130 (2), 160–180 (4)
190 (2), 180 (6)
160 (3), 180 (8)
160 (3), 180 (9)
0.8
1.0
1.1
1.6
0.9
0.9
0.50
2.04
0.05
0.04
0.59
0.03
1.11
0.03
0.03
0.06
0.06
84.5
92.02
80.5
79.3
85.0
75.2
89.1
76.8
86.1
86.8
90.2
1680
700
66.8
160.3
56.3
2.00 [16]
2.00 [17]
2.01 [18]
2.09 [19]
1.99 [20]
1.98 [21]
2.34 [22]
2.00 [23]
2.18 [24]
2.00
Zn(CH3COO)2
Ti(OBu)4
TiO(acac)2
KF/Al2O3
MgAl HT
Mg–Fe/Ti LDH 90–100 (5), 100 (1),180 (6) 1.2
Zn–Al LDH
MOF-5
2000
2600
1006
1011
3030
1610
3470
2208d
3100e
45.2
111.5
110.9
43.5
69.7
35.2
50.8
36.0
196 (4), 180–190 (6)
198 (3), 183 (3)
180 (2), 160 (4)
180 (3), 160 (4)
1.6
1.2
1.2
1.2
Yb(BTC)
Yb(BTC)
1.99
aReaction time
bReaction temperature
cAmount of catalyst accounting for mass percent of carbonate
dDiol is 1,4-butane diol
eDiol is 1,6-hexanediol
[31]. When MOF-5 was used as the catalyst, PCDL with the
highest number-average relative molecular mass and lowest
hydroxyl value was prepared. However, the hydroxyl func-
tionality of the product is far from 2, and the reusability of
MOF-5 is poor. When Yb(BTC) was used as the catalyst,
the amount of Yb(BTC) used in the reaction is less, and a
higher conversion was obtained in the transesterifcation.
The PCDL has a higher number-average relative molecu-
lar mass and lower hydroxyl value, and the hydroxyl func-
tionality of PCDL is close to 2, indicating that Yb(BTC)
is the most efcient catalyst for the synthesis of PCDL via
transesterifcation.
215
To confrm the Lewis acidity of Yb(BTC), TPD was
used to evaluate its interaction with pyridine (C5H5N). First,
100 mg of Yb(BTC) was placed in a sealed container flled
with C5H5N vapor for 1 h at room temperature so that C5H5N
was fully adsorbed on Yb(BTC). Next, Yb(BTC) was heated
at 100 °C for 30 min, and argon gas was used as the carrier
gas to remove the physically adsorbed C5H5N. Then, the
C5H5N-TPD curve of Yb(BTC) was obtained using a PCA-
1200 Chemisorption Analyzer with a heating rate of 10 °C/
min. The results are shown in Fig. 6. A desorption peak was
observed at 215 °C in the C5H5N-TPD curve of Yb(BTC),
indicating that the sample has moderate Lewis acidity and
can be expected to be a Lewis acid catalyst.
50 100 150 200 250 300 350 400 450
Temperature /oC
Fig. 6 C5H5N-TPD curve of Yb(BTC)
as Lewis acid active sites. A possible mechanism for the
synthesis of PCDL via the transesterifcation catalyzed by
Yb(BTC) is proposed in Fig. 7.
The carbonyl oxygen in DPC molecule interacts with the
Lewis acid active site on Yb(BTC) framework. The electrons
of carbonyl oxygen in DPC molecule are transferred to the
unsaturated metal ion Yb3+, which enhances the electrophi-
licity of carbonyl carbon in DPC molecule, and the carbonyl
carbon in DPC molecule is more susceptible to the attack
of nucleophilic species. Subsequently, the hydroxyl oxy-
gen in BDO molecule attacks the carbonyl carbon in DPC
molecules, generating an intermediate compound. Then,
the C–O bond is broken, and O–H bond is formed, yielding
Possible reaction mechanism
The 3D structure consists of seven-coordinated Yb3+ ions
with the pentagonal bipyramidal geometry and BTC ligands.
After the removal of terminal water molecules from the
channels, the resulting unsaturated Yb3+ centers can act
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Journal of the Iranian Chemical Society
Fig. 7 Proposed mechanism for the synthesis of PCDL via transterifcation catalyzed by Yb(BTC)
Table 2 Efect of catalyst amount on the synthesis of PCDL
Table 3 Reusability of Yb(BTC)
W/%
C/%
Mn
NOH/mg KOH/g
F
PDI
Run
C/%
Mn
NOH/mg KOH/g
F
PDI
0.01
0.03
0.06
0.08
0.10
57.4
66.3
86.6
86.8
86.9
1320
1517
2208
2207
2209
84.1
74.3
50.8
51.1
50.5
1.98
2.01
2.00
2.01
1.99
1.71
1.68
1.48
1.50
1.53
1
2
3
4
86.6
84.5
76.1
85.1
2208
2197
1882
2202
50.8
50.5
58.4
51.2
2.00
1.98
1.96
1.99
1.52
1.53
1.69
1.56
Table 2, when the amount of Yb(BTC) used in the reaction
is increased to 0.06% from 0.01%, the number-average rela-
tive molecular mass increases from 1320 to 2208 g/mol, and
the hydroxyl value decreases from 84.1 to 50.8 mg KOH/g.
Meanwhile, the PDI decreases to 1.48 from 1.71, indicating
that the distribution of number-average relative molecular
mass becomes narrower. However, further increase in the
amount of Yb(BTC) used in the reaction does not cause
an increase in number-average relative molecular mass
and decrease in the hydroxyl value. Unfortunately, further
increase in the amount of Yb(BTC) used in the reaction
increases the PDI. According to Table 2, suitable amount of
Yb(BTC) for the synthesis of PCDL via transesterifcation
between DPC and BDO is 0.06%.
phenol and 4-hydroxybutyl phenyl carbonate. The reaction
on the catalyst proceeds in this manner, so that PCDL is
synthesized.
Efect of catalyst amount
The efect of Yb(BTC) amount used in the reaction for the
synthesis of PCDL via the transesterifcation of DPC and
BDO was evaluated. As shown in Table 2, the amount of
Yb(BTC) used in the reaction plays a signifcant efect on
transesterifcation. The conversion of DPC increases with
the increase in the amount of Yb(BTC) used in the reaction
when the total Yb(BTC) mass fraction is less than 0.06%.
When the mass fraction exceeds 0.06%, further increase
in the amount of Yb(BTC) used in the reaction dose not
increase the conversion of DPC.
Reusability of catalyst
Reaction conditions: n(DPC) = 0.14 mol,
n(BDO)/n(DPC)=1.2. The transesterifcation temperature
and time were 180 °C and 2 h, respectively. The polycon-
densation was carried out at 160 °C and 9×103 Pa, and time
was 4 h.
Table 3 shows the reusability of Yb(BTC). When Yb (BTC)
was used for the third time, the conversion of DPC and
number-average relative molecular mass of PCDL evidently
decrease. Meanwhile, the hydroxyl value and PDI obvi-
ously increase, the distribution of number-average relative
molecular mass broadens, the hydroxyl functional degree
is below 2, and the performance of PCDL becomes poor.
The amount of Yb(BTC) used in the reaction also sig-
nifcantly afects the performance of PCDL. As shown in
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Journal of the Iranian Chemical Society
(B)
(A)
O
O
c
b
d
a
b
c
a
HO
OH
HO
OH
O
O
n
a
O
O
n
b
d
c
a
e
b
a
c
b
a
b
c
d
e
d
5
4
3
2
1
5
4
3
2
1
/ppm
/ppm
Fig. 8 1H NMR spectra of a PBC and b PHC
The catalytic activity of Yb(BTC) drastically decreases after
being used twice. The catalyst was recovered after the third
use, washed with dichloromethane for three times, and then
heated at 300 °C for 3 h. The catalytic activity of Yb (BTC)
is basically the same as that of the frst use.
and HOCH2CH2 groups. No peaks appeared at δ=7.30 and
1
δ = 7.42 on both the H NMR spectra. This indicates the
absence of phenyl terminal group in PBC and PHC, and the
decarboxylation of carbonate in the main chain of PCDL is
not obvious under the reaction conditions.
Reaction conditions: n(DPC) = 0.14 mol,
n(BDO)/n(DPC)=1.2. The transesterifcation temperature
and time were 180 °C and 2 h, respectively. The polycon-
densation was carried out at 160 °C and 9×103 Pa, and time
was 4 h.
Conclusions
A Yb-containing MOF Yb(BTC)(H2O)·(DMF) with a
high thermal stability was prepared using the solvother-
mal method. When Yb(BTC)(H2O)·(DMF) was calcined,
the coordinated water molecules and DMF guest molecules
could be easily removed from the channels to form a porous
solid Yb(BTC) with coordinatively unsaturated Yb3+ sites
and high BET surface area. The resulting solid exhibits
excellent catalytic activity in the synthesis of PCDL via
transesterifcation between DPC and diol.
Characterization of PCDL
The structure of PCDLs synthesized via transesterifcation
between DPC and diols was characterized by FTIR and 1H
NMR spectroscopy (CDCl3, ppm). The peak at~3455 cm−1
can be attributed to the stretching vibration of hydroxyl
group. The peaks observed at~2950 cm−1 and 2870 cm−1
correspond to the asymmetric and symmetric stretch-
ing vibration of methylene group, respectively. The peaks
at~1470 cm−1 and 1400 cm−1 can be attributed to the bend-
ing vibration of CH2 group. The peak at 1737 cm−1 repre-
sents the bending vibration stretching of carbonate C=O.
The peaks at 1336 cm−1 and 1247 cm−1 refect the stretching
vibration of aliphatic carbonate O–C–O.
Acknowledgements This work is fnancially supported by Chinese
Education Ministry Key Laboratory of Resource Chemistry, Shanghai
Key Laboratory of Rare Earth Functional Materials (Grant No. 2018),
Yunnan Provincial Applied Basic Research Program (2013FZ108),
China, National Innovation and Entrepreneurship Training Program for
College Students (201810684019), and Yunnan Provincial Innovation
and Entrepreneurship Training Program for College Students (2017).
As shown in Fig. 8a, the peaks observed at δ = 4.18
and δ = 1.72 correspond to the OC(O)OCH2 and OC(O)
OCH2CH2 groups on the main chain, respectively, and their
area ratio is 1:1, consistent with the ratio of theoretical pro-
ton number. The peaks at δ = 3.69 and δ = 1.65 represent
HOCH2 and HOCH2CH2 groups. Figure 8b shows that the
peaks at δ = 4.11, δ = 1.68, and δ = 1.42 correspond to the
OC(O)OCH2, OC(O)OCH2CH2, and OC(O)OCH2CH2CH2
groups on the main chain, respectively, and their area ratio
is 1:1:1, consistent with the ratio of theoretical proton num-
ber. The peaks at δ=3.67 and δ=1.56 refect the HOCH2
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