Indian Journal of Chemistry
Vol. 54A, May 2015, pp. 607-612
Layered double hydroxides based on different cations as catalysts for
synthesis of poly(1, 5-pentadiol) carbonate diols
Liping Wang*, Fan Wang & Linxiao Xu
College of Chemistry and Chemical Engineering, Qujing Normal University, PR China
Email: [email protected]
Received 13 October 2014; revised and accepted 6 April 2015
Layered double hydroxides (LDHs) based on different metal cations have been prepared by co-precipitation method and
characterized with X-ray diffraction and Fourier transform infrared spectroscopy. The sample LDHs have been used as
catalysts for transesterification between diphenyl carbonate (DPC) and 1, 5-pentadiol (1, 5-PD) to poly(1,5-pentadiol)
carbonate diols. The results show that Zn-Al-CO3 LDH and Mg-Al-CO3 LDH are the most effective catalysts for the
transesterification between DPC and 1,5-PD. Taking Zn-Al-CO3 LDH as an example, the effect of the layered structure of
LDHs on the transesterification has been investigated. The results show that the stability of layered structure is an important
factor that influences the catalytic activity of LDHs.
Keywords: Layered double hydroxides, Diphenyl carbonate, Transesterification, Pentadiol, Polycarbonate diols, Diols
Polycarbonate diol (PCDL) is a key material used in
the
synthesis
of
new
polycarbonate-based
polyurethane1. Compared with polyurethane prepared
from polyether or polyester diol, polyurethane
produced from PCDL shows better mechanical
performance, heat resistance, oil resistance, hydrolytic
resistance, oxidizing resistance and weather
resistance2.
The synthesis of PCDL via the transesterification
between organic carbonate and diol has received
considerable attention3. The transesterification
between dimethyl carbonate (DMC) and aliphatic diol
is regarded as a promising method for the preparation
of PCDL because DMC is a green chemical and
has been commercially produced via the oxidative
carbonylation of methanol instead of the conventional
phosgene route. However, due to its lower boiling
point (90 ºC) and ability to form azeotrope with
the methanol generated during the synthesis of
PCDL (azeotropic temp. 60 oC), DMC is easily
removed from the reactor along with methanol
at a high reaction temperature4 (160 oC). The synthesis
of PCDL via the reaction between diethyl carbonate
(DEC) and diols has been proposed because DEC
does not form azeotrope with the non-toxic product
ethanol
produced
during
the
preparation
of PCDL5. However, DEC can enter the body
through the respiratory system, causing poisoning.
In comparison, diphenyl carbonate (DPC) is
not only an environmentally friendly chemical
but has also been commercially produced by
non-phosgene methods6. Also, the product phenol
formed during the synthesis of PCDL can be recycled
to synthesize DPC by reacting with DMC. Hence,
the transesterification of DPC and aliphatic diols,
as shown in Scheme 1 (where R is
–CH2CH2CH2CH2CH2-) is a more promising green
route to prepare PCDL.
Layered double hydroxide (LDH), also called
as hydrotalcite (HT) or hydrotalcite-like compound,
is an anionic clay, with the general formula
[M2+(1-x)M3+x(OH)2]x+(An-)x/n·yH2O, where M2+ and
M3+ are respectively divalent and trivalent cations, and
An- is an exchangeable interlayer anion such as CO32-,
NO3-, Cl- etc7,8. The structure of hydrotalcite is based
upon layered double hydroxides with brucite like
(Mg(OH)2) hydroxide layers containing octahedrally
coordinated M2+ and M3+ cations. In recent years,
Scheme 1
608
INDIAN J CHEM, SEC A, MAY 2015
LDH has received considerable attention because
of its wide application in environmentally friendly
catalysis, molecular recognition and new organicinorganic hydrid composites9,10. In this work, LDHs
based on different divalent and trivalent cations were
prepared by co-precipitation method and used as the
catalysts in the transeterification between DPC and
1,5-pentadiol (1,5-PD).
Materials and Methods
Zinc nitrate hexahydrate (Zn(NO3)·6H2O, 99.0%,
Shanghai Xinbao Fine Chemical Plant), cupric nitrate
trihydrate (Cu(NO3)2·3H2O, 99.0%, Shanghai Huadong
Reagent company), cobaltous nitrate tetrahydrate
(Co(NO3)2·4H2O, 99.0%, Shanghai Xinbao Fine
Chemical Plant), nickel nitrate tetrahydrate
(Ni(CH3COO)2·4H2O, 99.0%, Shanghai Ruijie Reagent
Co.,
Ltd.),
aluminum
nitrate
nonahydrate
(Al(NO3)3·9H2O,
99.0%,
Shanghai
Kechang
Chemical Reagent Co., Ltd.), ferric nitrate
nonahydrate (Fe(NO3)3·9H2O, 99.0%, Shanghai
Runjie Chemical Reagent Co., Ltd.) and chromic
nitrate nonahydrate (Cr(NO3)3·9H2O, 99.0%,
Shanghai Runjie Chemical Reagent Co., Ltd.) were
used to prepared LDHs.
Diphenyl carbonate (DPC, 96.0%, Chongqing
Changfeng Chemical Plant) and 1,5-pentadiol (1,5-PD,
98.0%, Acros Organics Co., Ltd. ) were used to
prepare PCDL. All the reagents were used without
further purification.
Preparation of the catalyst and PDCL
A series of LDHs made from different metal ions,
where the molar ratio of divalent metal ion and
trivalent metal ion was 2, were prepared by the
co-precipitation method. A representative procedure
is as follows: The mixed aqueous solution (A)
containing the desirable amount of divalent metal
nitrate and trivalent metal nitrate and the mixed
aqueous solution (B) containing NaOH (2.00 g) and
Na2CO3 (3.30 g) were slowly added with stirring to
distilled water. The resulting suspension was stirred at
60 oC for 2 h and then filtered and washed with water
until the pH value of filtrate was near 7. The
precipitate was dried at 100 oC for 24 h and then
stored in a vacuum desiccator. The solids are
represented as Mg-Al-CO3 LDH, Mg-Fe-CO3 LDH,
Co-Al-CO3 LDH, Ni-Al-CO3 LDH, Cu-Al-CO3 LDH,
Zn-Fe-CO3 LDH, Zn-Cr-CO3 LDH, Zn-Al-CO3 LDH.
The synthesis of PCDL via the transesterification
between DPC and 1,5-PD includes two steps. During
the transesterification process, 0.14 mol DPC,
0.17 mol 1,5-PD and the catalyst were introduced into
a 100 mL flask equipped with a thermometer, a
mechanical stirrer and a reflux condenser. The reactor
temperature was gradually increased to 196 °C under
nitrogen atmosphere. This process was continued for
3.5 h until there was no formation of the side product
phenol, which was distilled off. During the
polycondensation process, the reactor temperature
was decreased to 180 °C and the pressure was reduced
to 6.0×104 Pa from normal pressure. These conditions
were maintained for 2 h. Eventually, the pressure was
reduced to 6.0×103 Pa for 4 h. The total reaction time
was about 9.5 h. After completion of the reaction, the
catalyst was separated from the liquid product PCDL
by filtration (when the temperature is 25 oC, the
product is a white solid).
Characterization
The prepared LDHs were characterized by powder
X-ray diffraction (PXRD), temperature programmed
desorption (TPD) and thermogravimetric/differentical
thermal analysis (TG-DTA). The PXRD measurements
were made on a Rigaku D/max 2500 PC powder
X-ray diffractometer using Cu Kα radiation, while
thermogravimetry-differential scanning calorimetry
data were collected on a SDT Q600 TA instrument.
The CO2-TPD and NH3-TPD curves of LDHs were
recorded on a Micromeritics Chemiorption Ananlyzer.
The product was analyzed by gel permeation
chromatography (GPC), Fourier transform infrared
(FTIR) spectroscopy and NMR spectroscopy. FTIR
spectrum of the product was recorded on a Nicolet
560 FT-IR spectrometer. The 1H NMR and 13C NMR
spectra of the product were recorded on a Bruker
Avance III 500 MHz instrument using deuterated
chloroform (CDCl3) as the solvent. The molecular
weight analysis of the product was carried out on a
WATERS gel permeation chromatograph in DMF
solution and the calibration curve was made using a
standard sample of mono-dispersed polystyrene.
A Shimadzu GC-2010 gas chromatograph
equipped with a flame-ionization detector was used to
qualitatively and quantitatively analyze the distillate.
Results and Discussion
Characterization of LDHs
The LDHs based on different metal cations
were prepared by co-precipitation method and
characterized by X-ray diffraction (XRD) (Fig. 1). As
shown in Fig. 1, seven LDHs samples also exhibit the
WANG et al.: LAYERED DOUBLE HYDROXIDES AS CATALYSTS FOR SYNTHESIS OF POLYCARBONATE DIOLS
diffraction peaks (003), (006), (009) and (110) which
present the hydrotalcite-like structure.
Catalytic activity of LDHs
During the transesterification process, the activity
of catalyst is evaluated by the yield of phenol (y),
while the number-average molecular weight (Mn) and
the hydroxyl value of PCDL are used to determine the
activity of catalyst in the polycondensation process.
The catalytic activities of different LDHs for the
synthesis of PCDL via the transesterification between
DPC and 1,5-PD were investigated, and the results are
shown in Table 1.
As shown in Table 1, all the samples exhibit
catalytic activity for the transesterification. When
Fig. 1 – X-ray diffraction spectra of different LDHs. [(1) Zn-AlCO3 LDH, (2) Zn-Cr-CO3 LDH; (3) Zn-Fe-CO3 LDH;
(4) Ni-Al-CO3 LDH; (5) Co-Al-CO3 LDH; (6) Mg-Fe-CO3 LDH;
(7) Mg-Al-CO3 LDH].
609
Zn-Al-CO3 LDH and Mg-Al-CO3 LDH were used as
the catalysts, the yield of phenol is higher than when
other LDHs are used as the catalyst. At the same time,
the products have higher Mn and lower hydroxyl
value. Thus, Zn-Al-CO3 LDH and Mg-Al-CO3
LDH are more efficient catalysts for the synthesis of
PCDL via the transesterification between DPC and
aliphatic diol.
LDH have acid and basic sites, and the change in
the chemical composition and activation conditions
may strengthen or weaken the acidity and basicity of
LDH11,12. The transesterification is a reaction
catalyzed by acid or base. The different catalytic
activities of LDHs may be related to their different
acidity and basicity. The basicity of LDH is derived
from the interlayer anions and the nature of the M-O
band on the layer. Due to the presence of the same the
interlayer anions CO32-, the basicity of LDH mainly
depends on the nature of the M-O band. The
electronegativity of M is weak, and the M-O band is
easy to break, so the basicity of LDH is stronger.
The electronegativity values are 1.208 (Mg2+),
1.428 (Zn2+), 1.467 (Co2+), 1.786 (Ni2+), 1.517 (Cu2+),
1.499(Al3+), 1.687 (Fe3+) and 1.661 (Cr3+)13. As
shown in Table 1, Mg-Al-CO3 LDH and Zn-Al-CO3
LDH with stonger basicity indicate higher catalytic
activity for the synthesis of PCDL.
The Lewis acidity of LDH mainly depends on the
divalent and trivalent metal ions in the octahedral
interstices of hydroxide layer. When LDHs have the
same molar ratio of divalent cation to trivalent cation
and the same interlayer anion, the catalytic activity of
LDH may be related to the property of divalent and
trivalent metal ions11,12,14-16. Zhang17 reported that the
Table 1 – Effect of different LDHs on transesterification between DPC and 1,5-PD
Catalyst
y (%)
Mn
Hydroxyl value
(mg KOH/g)
62.9
95.1
68.1
96.1
100.7
-
Polydispersity
Zn-Al-CO3 LDH
80.4
1765
1.71
Zn-Al-CO3 LDHa
64.7
1174
2.25
Mg-Al-CO3 LDH
78.3
1625
1.80
Co-Al-CO3 LDH
71.4
1150
2.31
Ni-Al-CO3 LDH
69.2
1100
2.30
Cu-Al-CO3 LDH
64.4
Zn-Fe-CO3 LDH
62.3
Mg-Fe-CO3 LDH
61.3
Zn-Cr-CO3 LDH
53.5
a
Molar ratio of Zn2+/Al3+ is 3.
React. cond.: n(DPC)=0.14 mol; n(1,5-PD):n(DPC) = 1.2; w(catalyst) = 0.03% (the amount of catalyst is based on the amount of DPC),
reaction temp. is 196 ºC in the transesterification process; reaction time: 3.5 h; reaction temp. is 180 ºC in the polycondensation process;
Pressure is reduced to 6.0×104 Pa for 2 h, then the pressure is reduced to 6.0×103 Pa for 4 h.
610
INDIAN J CHEM, SEC A, MAY 2015
Lewis acid strengths of metal ions are related to
electrostatic force and the electronegativity of elements
in valence states. The Lewis acid strengths are
respectively 1.402 (Mg2+), 0.656 (Zn 2+), 0.356 (Co2+),
0.2993 (Ni2+), 0.177 (Cu2+), 3.042(Al2+), 1.311 (Fe3+
and Cr3+). From Table 1, it can be clearly seen that the
catalytic activity of LDHs is related to the Lewis acid
strengths of trivalent metal ions. Among these LDHs,
the LDHs made from Al3+ with the strongest Lewis
acid strength exhibit the highest catalytic activity, and
the LDHs prepared from Cr3+ with the weakest Lewis
acid strength exhibit the lowest catalytic activity. For
the LDHs with the same trivalent metal, the Lewis
acid strengths of divalent metal ions also influence the
catalytic activities of LDHs. With the increase of
Lewis acid strengths of divalent metal ions, the
catalytic activities of LDHs are also enhanced.
However, when Lewis acid strengths of divalent
metal ions are above 0.6, Lewis acid strengths of
divalent metal ions have an insignificant impact on
the catalytic activities of LDHs for the the synthesis
of PCDL via the transesterification between DPC and
1,5-PD. Of these, LDHs produced from Zn2+ and Al3+
is the most efficient catalyst for the transesterification
between DPC and 1,5-PD.
The proposed mechanism is as follows: The
carbonyl oxygen atom in DPC combined with acid
site of LDH enhances the electrophilicity of carbonyl
carbon atom. Besides, the hydroxyl in 1,5-PD is
polarized by the basic site of LDH and generates a
nucleophic species, HOC5H10O-, that attacks the
carbonyl carbon atom in DPC, displacing the
phenoxyl group C6H5O-. Then the phenoxyl group
C6H5O- combines with hydrogen on the LDH,
yielding phenol. In this manner, the transesterification
proceeds continuously on the LDH catalyst and PCDL
is finally synthesized by transesterification followed
by polycondensation.
n(Zn)/n(Al) = 2 is the more efficient catalyst for the
preparation of PCDL.
CO2-TPD curves of Zn-Al-CO3 LDHs are shown in
Fig. 2(a). The increase in the molar ratio of Zn2+ and
Al3+ improves the basicity of Zn-Al-CO3 LDH. The
acidity of Zn-Al-CO3 LDHs has been characterized by
NH3-TPD (Fig. 2b). As shown in Fig. 2(b), there are
two desorption peaks in the each NH3-TPD curve,
suggesting that each LDH has two different acid sites.
With the increase of molar ratio of Zn2+ and Al3+, the
strength of acidity reduces obviously. Combined with
data in Table 1, the catalytic activity is obviously
related to acidity of Zn-Al-CO3 LDHs.
TG-DTA curves of Zn-Al-CO3 LDHs with
n(Zn)/n(Al) = 2 and n(Zn)/n(Al) = 3 are shown in
Fig. 3(a) and 3(b) respectively. There is a peak at
205 ºC on the TG-DTA curve of Zn-Al-CO3 LDH
with n(Zn)/n(Al) = 2, while the TG-DTA curve of
Zn-Al-CO3 LDH with n(Zn)/n(Al) = 3 shows a peak at
182 ºC. When the temperature is 205 ºC, Zn-Al-CO3
LDH with n(Zn)/n(Al) = 2 loses the interplay water,
the hydroxyl group in the brucite-like sheet and the
interplay anion CO32-, while Zn-Al-CO3 LDH with
n(Zn)/n(Al) = 3 loses the interplay water, the
hydroxyl group in the brucite-like sheet and the
interplay anion CO32- when the temperature reaches
182 ºC. It is clearly that below the reaction
temperature (196 oC), the stablity of Zn-Al-CO3 LDH
with n(Zn)/n(Al) = 2 is better than that of Zn-Al-CO3
LDH with n(Zn)/n(Al) = 3. Hence, stability of the
layered structure is an important factor that influences
the catalytic activity of LDHs for the
transesterification between DPC and 1,5-PD.
Spectral data of PCDL
The FTIR spectrum of PCDL (Fig. 3): the bands at
3500 cm-1 and 3490 cm-1 are due to the absorption
Effect of cations molar ratio on the catalytic activities of LDHs
Zn-Al-CO3 LDHs with n(Zn)/n(Al) = 2 and
n(Zn)/n(Al) = 3 were used as the catalyst for the
transesterification between DPC and 1,5-PD, and the
results are listed in Table 1.
As shown in Table 1, Zn-Al-CO3 LDH with
n(Zn)/n(Al) = 2 exhibits higher catalytic activity than
with n(Zn)/n(Al) = 3 in the transesterification process.
Meanwhile, the resulting product PCDL has higher
number average molecular weight and lower hydroxyl
value when Zn-Al-CO3 LDH with n(Zn)/n(Al) = 2 is
used as the catalyst, indicating that LDH with
Fig. 2 – (a) CO2-TPD and (b) NH3-TPD curves of Zn-Al-CO3
LDHs. [(1) Zn-Al-CO3 LDH with n(Zn)/n(Al) = 2; (2) Zn-Al-CO3
LDH with n(Zn)/n(Al) = 3].
WANG et al.: LAYERED DOUBLE HYDROXIDES AS CATALYSTS FOR SYNTHESIS OF POLYCARBONATE DIOLS
611
Fig. 3 – (a) TG-DTA curve of LDH with n(Zn)/n(Al) = 2, and, (b) TG-DTA curve of LDH with n(Zn)/n(Al) = 3.
of -OH, the bands at 2967 cm-1, 2900 cm-1, 2877 cm-1,
1473 cm-1 and 1405 cm-1 are the characteristic
absorptions of CH2, the band at 1751 cm-1 is the
absorption of carbonate C=O, while the bands at
1267 cm-1 and 1220 cm-1 are the absorption of
aliphatic carbonate O-C-O (Supplementary Data,
Fig. S1).
The product was also characterized by 1H NMR
and 13C NMR (Supplementary Data, Fig. S2). The
peaks at 4.23, 1.63, 1.60, 1.52 ppm correspond to the
protons in the OC(O)OCH2, OC(O)OCH2CH2,
OC(O)OCH2CH2CH2 and HOCH2CH2, respectively.
The ratio of these peak areas is 1:1:1, since the area of
each peak is proportional to the number of proton in
the related group. The peak at 3.68 ppm represents the
proton in the CH2 adjacent to OH end group. There
are no peaks at 7.30, 7.41 and 7.27 ppm
corresponding to the protons in the terminal phenyl
group in the 1H NMR spectrum, indicating that
there is no terminal phenyl group in the molecular
chain of PCDL.
In the 13C NMR spectrum, the peak at 155.3 ppm is
attributed to the C=O, the peak at 67.8 ppm reflects
the CH2 adjacent to the carbonyl group C=O, while
the peak at 62.7 ppm corresponds to the CH2 adjacent
to OH end group. The peaks at 31.9, 28.9 and
28.1 ppm are attributed to the HOCH2CH2,
C(O) OCH2CH2 and OHCH2CH2CH2, respectively.
There are no peaks at 121.8, 126.0, 129.3 and
151.2 ppm corresponding to the terminal phenyl
group in the 13C NMR spectrum, indicating that
there is no terminal phenyl group in the molecular
chain of PCDL. These data verify that the product
is PCDL.
Conclusions
Among the studied seven LDHs, Zn-Al-CO3 LDH
and Mg-Al-CO3 LDH exhibit more effective catalytic
activities for the synthesis of PCDL via the
transesterification between DPC and 1,5-PD. Taking
Zn-Al-CO3 LDH as an example, studies on the effect
of the layered structure of LDHs on the
transesterification show that the stability of layered
structure is an important factor that influences the
catalytic activity of LDHs.
Supplementary Data
Supplementary data associated with this article, viz.,
Figs. S1 and S2, are available in the electronic form
at http://www.niscair.res.in/jinfo/ijca/IJCA_54A(05)607612_SupplData.pdf.
Acknowledgement
This work was supported by Yunnan Provincial
Applied Basic Research Program (2013FZ108),
China.
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