Conversion of biomass-derived ethyl levulinate into γ-valerolactone via hydrogen transfer from supercritical ethanol over a ZrO2 catalyst

Article abstract of DOI:10.1039/c3ra41288a

A green and efficient process was developed for the conversion of biomass-derived ethyl levulinate (EL) into γ-valerolactone (GVL) using supercritical ethanol as the hydrogen donor and the reaction medium over low-cost and eco-friendly ZrO₂ catalysts, which were prepared by the precipitation method and characterized by BET, SEM, XRD, FT-IR, TGA-DTA, TPD-NH₃ and TPD-CO₂. The results indicated that amorphous ZrO₂ with a high specific surface area and a large number of acid-base sites exhibited the highest catalytic activity, an excellent GVL yield of 81.5% with 95.5% EL conversion was achieved at 523 K over 3 h. In addition, combined with the results of poisoning experiments, a plausible mechanism of catalytic transfer hydrogenation (CTH) via a six-membered ring transition state was also presented. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41288a

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Conversion of biomass-derived ethyl levulinate into
c-valerolactone via hydrogen transfer from supercritical
Cite this: RSC Advances, 2013, 3,
1
0277
ethanol over a ZrO catalyst3
2
Xing Tang, Lei Hu, Yong Sun,* Geng Zhao, Weiwei Hao and Lu Lin*
A green and efficient process was developed for the conversion of biomass-derived ethyl levulinate (EL)
into c-valerolactone (GVL) using supercritical ethanol as the hydrogen donor and the reaction medium
over low-cost and eco-friendly ZrO
characterized by BET, SEM, XRD, FT-IR, TGA-DTA, TPD-NH
amorphous ZrO with a high specific surface area and a large number of acid–base sites exhibited the
2
catalysts, which were prepared by the precipitation method and
3
and TPD-CO . The results indicated that
2
Received 7th January 2013,
Accepted 10th April 2013
2
highest catalytic activity, an excellent GVL yield of 81.5% with 95.5% EL conversion was achieved at 523 K
over 3 h. In addition, combined with the results of poisoning experiments, a plausible mechanism of
catalytic transfer hydrogenation (CTH) via a six-membered ring transition state was also presented.
DOI: 10.1039/c3ra41288a
www.rsc.org/advances
inevitably increase production costs in prospective applica-
tions. Furthermore, the production of LA is an energy-
intensive process due to its high boiling point. Taking the
above-mentioned problems into consideration, development
of a green and low-cost approach for the synthesis of GVL is
urgently needed.
Introduction
With the gradual depletion of fossil resources and the
continued deterioration of environmental quality, the search
for renewable resources is critically important. Among the
various renewable resources, biomass, which is widespread,
abundant and inexpensive, is regarded as an excellent
substitute for nonrenewable fossil resources. In recent years,
a promising approach for the utilization of biomass has been
In contrast to LA, the low boiling point, acid-free and easily-
separable EL, produced by the ethanolysis of various carbohy-
drates such as glucose, fructose, sucrose, or cellulose, is a
1
–6
to produce various chemicals. Among these chemicals, GVL,
which is identified as a versatile intermediate for the
production of high value-added chemicals and high perfor-
mance liquid fuels (Scheme 1), has attracted worldwide
31
better alternative for the synthesis of GVL. Recently, Zhao
32
et al.
demonstrated a high-pressure direct liquefaction
process through supercritical ethanolysis over metal oxides,
GVL was detected in the mixture after reaction. Dumesic et al.
reported the catalytic transfer hydrogenation (CTH) of LA and
its esters into GVL through the Meerwein–Ponndorf–Verley
7
–15
attention.
In a considerable number of studies in this field, GVL is
obtained by the hydrogenation of levulinic acid (LA), a
precursor that can be derived from the hydrolysis of various
carbohydrates such as glucose, fructose, sucrose, or cellulose.
Generally, the hydrogenation of LA is driven by external
(
MPV) reaction over metal oxide catalysts using alcohols as the
33
hydrogen donors in the presence of an alkylphenol.
However, the elevated additional pressure with He (300 psi)
and the prolonged reaction time (8–16 h) were prerequisite for
ensuring a high yield of GVL. Moreover, the introduction of
alkylphenol will complicate the subsequent separation step of
GVL.
molecular H
Ir).
2
over noble metal catalysts (such as Rh, Pt, Ru,
In addition, the hydrogenation of LA can also be
driven by internal molecular H derived from the decomposi-
tion of formic acid (FA) over supported gold catalysts or
1
6–26
2
Herein, the conversion of EL into GVL via CTH in
supercritical ethanol without any external gas was reported.
A series of low-cost and environmentally benign ZrO catalysts
2
was prepared and characterized, and various reaction para-
meters were investigated in order to obtain a higher yield of
GVL.
2
7–30
immobilized Ru catalysts.
Although good yields of GVL
were obtained, these catalysts used for the hydrogenation of
LA or the decomposition of FA are very expensive and easily
deactivated. Moreover, LA and FA are very corrosive, which will
School of Energy Research, Xiamen University, Xiamen 361005, China.
E-mail: sunyong@xmu.edu.cn; lulin@xmu.edu.cn; Fax: +86-0592-5952786;
Tel: +86-0592-5952786
3
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3ra41288a
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Scheme 1 The pathway for the conversion of GVL into chemicals and fuels.
growth and perfection of ZrO crystals, which correspondingly
results in the increase of the particle sizes and the decrease of
the specific surface areas (Table 1).
The CO -TPD and NH -TPD measurements were performed
2 3
to characterize the acid–base properties of the catalyst, the
results are presented in Fig. 2 and 3, respectively.
The CO -TPD profile of ZrO -573 exhibits a significant
2 2
desorption peak at 416 K and a small desorption peak at 650 K,
which can be assigned to the medium basic sites and the
Results and discussion
2
In order to study the catalysis of ZrO
EL, the detailed characterization of the ZrO
at different calcination temperatures was carried out.
2
solids during the CTH of
catalysts obtained
2
XRD patterns and the textural properties of ZrO catalysts
2
are given in Fig. 1 and Table 1, respectively. It can be seen that
ZrO -573 and ZrO -773 existed as an amorphous phase and as
2
2
a mixture of a monoclinic phase and a tetragonal phase,
respectively, which is confirmed from the results of Djurado
strong basic sites where CO
bidentate carbonate and unidentate carbonate, respectively
Scheme 2), and these results are in line with the study of
2
is adsorbed in the form of
3
4
35
et al. and Kanade et al. When the calcination temperature
was further increased to 973 K, only a monoclinic phase is
(
present in the ZrO
2
catalyst and its diffraction peaks become
36
Urbano et al. Moreover, it is obvious that medium basic sites
are dominant in ZrO -573. Compared to ZrO -573, there are
fewer medium basic sites on the surface of ZrO -773, and no
evident desorption peak is observed in the CO -TPD profile of
more intense and sharp. Elevated temperatures facilitate the
2
2
2
2
2 3
ZrO -973. Similarly, a broad NH desorption peak that can be
assigned to the medium acidic sites is observed at around 545
K in the NH -TPD profile of ZrO -573, and NH desorption
3
2
3
peaks in the profiles of ZrO
2
-773 and ZrO
2
-973 are not
apparent.
In the profiles of TPD, the number of acid–base sites is
reflected by the peak areas. As can be seen from Fig. 2 and 3,
2
Table 1 Specific surface area and textural properties of ZrO obtained at
different calcination temperatures
a
2
21
b
b
Solid
S
BET (m g
)
Particle size (nm)
Crystallinity (%)
ZrO
ZrO
ZrO
2
2
2
-573
-773
-973
157.2
40.6
27.3
Amorphous
12
23
Amorphous
72.3
97.7
a
b
Fig. 1 XRD patterns for (a) ZrO
2
-573; (b) ZrO
2
-773; (c) ZrO
2
-973; m = monoclinic
Determined by BET analysis of N
2
adsorption isotherms. As XRD
determined, where m = monoclinic phase; t = tetragonal phase.
phase; t = tetragonal phase.
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adsorbed water, and indicated that ZrO
2
-573 with a high
specific area can adsorb much water. Moreover, the mass drop
of ZrO -573 occurred continuously with increasing tempera-
ture after 373 K, and a sharp exothermic peak appeared at
around 725 K (Fig. S2, ESI ). The exothermic event is attributed
to a release of the hydroxyl groups of the remaining Zr-OH
species in the incomplete ZrO lattice, resulting in the
formation and crystallization of the amorphous zirconia.
Furthermore, the mass loss of ZrO -573 did not exceed 4 wt%
when the temperature was increased beyond 373 K, which
indicated that only a small percentage of defective ZrO lattice
existed in ZrO -573. In conclusion, the OH bands in the FT-IR
2
3
2
3
7
2
2
2
spectrum were a result of both remaining Zr-OH species and
water adsorbed on the surface of the catalyst.
SEM images show that both fresh and spent ZrO
consisted of primary crystallites (Fig. 5). Most of these primary
crystallites are smaller than 10 nm in ZrO -573 (Fig. 5(a)),
indicating ZrO -573 has a large specific surface area, which is
2
catalysts
2 2 2 2
Fig. 2 CO -TPD profiles for (a) ZrO -573, (b) ZrO -773 and (c) ZrO -973.
2
2
in agreement with the results in Table 1. However, the
diameter of primary crystallites increased to 30–40 nm and
there are many more acidic sites and basic sites in ZrO
2
-573
than in ZrO -773 and ZrO -973, respectively, which may be
2
2
2 2
50–100 nm in ZrO -773 and ZrO -973, respectively. In line with
attributed to the lower specific surface areas of ZrO
at 773 and 973 K leading to fewer acid–base sites.
2
calcined
the results of the XRD, the SEM images also indicate that the
crystallites become bigger and bigger with an increase in
calcination temperature. Moreover, the size of the primary
crystallites and the surface morphology of the catalysts were
apparently not changed before and after use.
Given the wide differences in the results of the catalyst
characterizations, the CTH of EL was performed over various
catalysts in supercritical ethanol (Table 2, entries 8, 11 and 12).
The results from FT-IR spectroscopy of ZrO
shown in Fig. 4. The broad band at 3400–3500 cm is due to
2
catalysts are
2
1
2
1
the stretching vibration of the hydroxyl. The peak at 742 cm
3
5
is representative of monoclinic zirconia. The band at 1621
2
1
cm is assigned to O–C–O stretching modes of CO
2
absorbed
2
1
from the atmosphere. The band at 1330 cm is attributed to
bidentate carbonates (Scheme 2). However, no obvious peak
is observed at 1330 cm in the FT-IR spectrum of ZrO -973,
2
3
7
2
1
ZrO
2
-573 was the most active. Compared with ZrO
2
-573, the
catalytic activities of ZrO
2
-773 and ZrO -973 were very poor,
2
which may be ascribed to ZrO -973 having no obvious acid–
2
leading to less than 30% GVL yield and 35% EL conversion,
respectively, which is probably due to there being fewer acid–
base sites on their surface.
base sites.
In addition, the thermogravimetric analysis of the catalysts
was conducted and the results are given in Fig. S1 and S2, ESI.
In contrast to ZrO -773 and ZrO -973, a significant mass loss
3
Then, based on the good performance of the amorphous
2
2
ZrO
2
, the conversion of EL into GVL was conducted in the
-573 as the catalyst (Table 2).
(
about 4 wt%) occurred when the ZrO -573 was heated from
2
presence of ethanol using ZrO
2
293 to 373 K. This can be assigned to the loss of physically
When the reaction temperature was 453 K, only a 23.4% GVL
yield and a 35.9% EL conversion were achieved. When the
reaction temperature was elevated to 523 K, the reaction
pressure approached 70 bar, which is in the supercritical state
c c
of ethanol (T = 516.2 K, P = 63.8 bar). Under the present
conditions, a 62.5% yield of GVL and an 81.5% conversion of
EL were obtained. Moreover, 43.0% GVL yield and 55.6% EL
conversion were still achieved when the substrate concentra-
tion was increased to 10 wt% (Table 2, entry 9). When the
reaction temperature was further increased to 533 K, EL
conversion was correspondingly improved to 83.7%, however,
the GVL yield dropped down to 60.8%, indicating that more
byproducts were formed due to the extra-high reaction
temperature. Therefore, moderate reaction temperature of
523 K was chosen for the conversion of EL into GVL in the
subsequent experiments. Besides ethanol, other alcohols were
also used as the hydrogen donor for the reduction of EL (Table
S1, ESI ). The corresponding levulinates, other than EL,
3
Fig. 3 NH
3
-TPD profiles for (a) ZrO
2
-573, (b) ZrO
2
-773 and (c) ZrO
2
-973.
formed by transesterification when another alcohol replaced
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2 2
Scheme 2 Three types of interactions of CO with basic sites on the surface of ZrO .
ethanol as the solvent and the catalysts seem to become more
active in isopropanol and 1-butanol media.
the EL conversion were observed. From the viewpoint of
economy, the amount of 1 g was selected as an appropriate
catalyst loading.
In addition, it should be pointed out that ethanol as a
hydrogen donor is much cheaper and safer than
H
2
.
The long-term stability and reusability of the catalyst are
extremely important considerations for the practical conver-
sion of EL into GVL to reduce the manufacturing cost.
Meanwhile, the unreacted ethanol can be easily recovered
and reused. Another advantage of CTH in the presence of
ethanol is that the pressure spontaneously caused by the
swelling of ethanol under elevated temperature is sufficient to
maintain the supercritical state of ethanol. Hence, no
additional gas needs to be introduced into the reaction system.
The EL conversion was also strongly related to the reaction
time. As illustrated in Fig. 6, a yield of only 28.0% GVL and a
Therefore, in this work, the recyclability of ZrO -573 was
2
2
tested. After the first reaction cycle, ZrO -573 was separated
from the reaction mixture by filtration without any other
further treatment, and then reused in the next cycle under the
same reaction conditions. As can be seen from Fig. 7, an
evident decrease in the EL conversion from 81.5 to 64.9% and
GVL yield from 62.5 to 50.1% was observed in the second cycle.
The conversion of EL and the yield of GVL gradually dropped
39.1% EL conversion were obtained after 0.5 h. The yield of
GVL was further increased and then fell when the reaction
time was increased from 1 to 4 h, however, EL conversion was
enhanced continuously, which indicates that more and more
undesired reactions, such as aldol condensation between EL,
GVL and aldehydes (derived from the dehydrogenation of
ethanol) occurred when the reaction time exceeded 3 h,
leading to a decrease in the yield of GVL.
down to 54.6 and 43.3% when ZrO -573 was used 4 times,
2
respectively. The decrease was possibly due to the partial
deactivation of ZrO -573 caused by the deposition of humins
2
(
probably formed by an aldol condensation between EL, GVL
and an aldehyde) on the surface of ZrO -573.
Fortunately, the deactivated ZrO -573 could be readily
2
2
The effects of the dosage of the catalyst on the conversion
regenerated via simple calcination at 573 K for 4 h. The EL
conversion of 77.2% and GVL yield of 58.4% obtained when
the regenerated ZrO -573 was used in the fifth cycle were
2
parallel to those obtained in the first cycle. The characteriza-
tion of the spent catalyst was also conducted, and the results
of EL into GVL are given in Table 3. In the absence of ZrO -573,
2
the GVL yield is negligible. When 0.3 g ZrO -573 was used, a
2
GVL yield of 37.7% and a 46.0% conversion of EL were
observed. Increasing the amount of ZrO
resulted in a remarkable increase in the yield of GVL, from
6.2 to 62.5%, and the conversion of EL from 54.6 to 81.5%,
respectively. When the amount of ZrO -573 was further
increased to 1.2 g, only slight increases in the GVL yield and
2
-573 from 0.5 to 1 g
are given in the ESI.
surface area of the spent catalyst was decreased slightly (Table
S2, ESI ). Meanwhile, there was not a noticeable difference
3
Compared with the fresh catalyst, the
4
2
3
between the surface area of the spent catalysts and the
regenerated catalyst. However, an obvious drop in EL conver-
sion and GVL yield were observed after the fourth reaction
2
cycle, and the comparable catalytic activity of ZrO -573 was
recovered after regeneration. The above-mentioned results
show that the loss of the activity of spent ZrO -573 was largely
2
due to humins deposited on the surface of the ZrO
regeneration process removed the humins and recovered the
catalytic activity. It is noteworthy that the reactivated ZrO -573
catalyst was totally converted to the tetragonal phase from the
amorphous phase (Fig. S3, ESI ), although the size of primary
crystallites and surface morphology of the catalysts were not
changed evidently before and after regeneration (Fig. S4, ESI ).
2
-573, the
2
3
3
Poisoning experiments were also carried out by introducing
extra additives into the reaction system, and the results are
depicted in Fig. 8. In the presence of pyridine, the catalytic
activity of ZrO -573 was slightly decreased. It is probable that
2
the poor adsorption performance of pyridine on the surface of
3
8
Fig. 4 FT-IR patterns for (a) ZrO
2
-573, (b) ZrO
2
-773 and (c) ZrO
2
-973.
the catalyst led to the weak poisoning effect. However, the
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2 2 2 2
Fig. 5 SEM images for (a) ZrO -573, (b) ZrO -773, (c) ZrO -973 and (d) ZrO -573 recovered after cycle 4.
addition of benzoic acid drastically reduced the conversion of
EL. This can be explained by the strong interaction between
benzoic acid and base sites. Meanwhile, it should be noted
ZrO -573, 78.4% EL conversion and a 59.8% GVL yield were
still obtained.
The above mentioned poisoning experiments revealed that
the process of CTH for the conversion of EL into GVL was
2
that ZrO -573 was very sensitive to sulfuric acid and sodium
2
hydroxide. A significant decrease in EL conversion and GVL
yield was observed when sulfuric acid or sodium hydroxide
was added. Furthermore, the addition of small amounts of
water did not substantially change the catalytic activity of
2
closely related to the acid–base property of ZrO . Combined
with the results of the catalyst characterization, a plausible
mechanism of CTH analogous to that presented by Ivanov
3
8
et al. was presented (Scheme 3). Hydrogen transfer is a
concerted process that takes place via a six-membered ring
transition state formed between ethanol and EL. The rate-
determining step of the process must be related to the
interaction of the alcohol with the acid–base sites (unsaturated
a
Table 2 Conversion of EL into GVL under various conditions
4+
22
Zr –O
pairs), which causes its dissociation to the corre-
sponding alkoxide. The surface-adsorbed alkoxide transfers a
hydride ion that attacks the carbonyl group of EL to yield ethyl
4-hydroxypentanoate (4-HPE), and then the 4-HPE is further
converted into GVL through an intramolecular transesterifica-
tion accompanied by the equimolar production of ethanol.
Meanwhile, the ethanol is converted into the corresponding
acetaldehyde after losing two hydrogens, then the latter
condensed with two ethanol molecules to afford acetal.
Although acetal was determined as the second product to
GVL by a GC-MS analysis, no detectable 4-HPE was observed.
This result indicated that the rate of intramolecular transes-
terification of 4-HPE is very fast under the reaction tempera-
ture. In addition, some derivatives of GVL were identified from
b
Entry Tem. (K) Pressure (bar) GVL yield (%) EL conversion (%)
1
2
3
4
5
6
7
8
9
1
1
1
453
463
473
483
493
503
513
523
523
533
523
523
20
24
30
36
42
52
60
70
70
86
70
70
23.4
24.3
26.4
52.1
54.1
59.0
61.8
62.5
43.0
60.8
28.6
12.7
35.9
38.7
41.6
65.2
66.5
70.4
74.0
81.5
55.6
83.7
32.1
14.3
c
0
1
2
d
e
a
2
Reaction conditions: EL, 2 g; ZrO -573 catalyst, 1 g; ethanol, 38 g;
reaction time, 1 h. Reaction pressure is obtained by the swelling of
ethanol itself at the specified temperature. 4 g EL as feed. ZrO
2
73 as the catalyst. ZrO -973 as the catalyst.
b
c
d
2
-
3
a GC-MS analysis (Scheme S1, ESI ), but the reaction
mechanism involving these derivatives is not very clear.
e
7
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Fig. 6 Effect of reaction time on the conversion of EL into GVL. Reaction
Fig. 7 Recycling of the catalysts in the conversion of EL into GVL. Reaction
conditions: EL, 2 g; ZrO -573, 1 g; ethanol, 38 g; reaction temperature, 523 K;
2
conditions: EL, 2 g; ZrO -573, 1 g; ethanol, 38 g; reaction temperature, 523 K.
2
reaction time, 1 h. Catalyst regeneration: calcination at 573 K for 4 h after cycle 4.
Further understanding of these minor-products is essential
and will be the focus of future studies, however this issue is
not of primary concern in this paper.
Experimental procedure
Materials
EL (98%) and GVL (98%) were purchased from Alfa Aesar Co.
Ltd. (Tianjin, China). Zirconium oxychloride (99%) was
obtained from Aladdin Reagent Co. Ltd. (Shanghai, China).
All other chemicals were supplied from Sinopharm Chemical
Reagent Co. Ltd. (Shanghai, China) and used without further
purification.
Conclusions
In the present work, a series of ZrO catalysts were prepared,
2
characterized and used for the synthesis of GVL from biomass-
derived EL in the presence of supercritical ethanol that could
be employed as the hydrogen source and reaction solvent
Catalysts preparation
simultaneously. Among various ZrO
2
catalysts, amorphous
ZrOCl ?8H O was dissolved in deionized water to prepare a 100
g L ZrOCl ?8H O solution. Concentrated NH OH was added
2 2 4
ZrO was found to be the most active, a yield of GVL and a
conversion of EL as high as 81.5 and 95.5% were achieved at
2
2
2
2
1
to adjust the pH value to between 9 and 10 with vigorous
stirring, and then aged at room temperature for 24 h. The
obtained precipitate was thoroughly washed with deionized
water until chloride ions could not be identified in the filtrate.
The washed precipitate was dried at 383 K for 12 h, and then
5
2
23 K for 3 h, respectively. Fortunately, the ZrO -573
regenerated through simple calcination still had excellent
stability and reusability after the fifth cycle. Moreover, a
mechanism for the conversion of EL into GVL via CTH was
also proposed according to the results of the catalyst
characterization and the poisoning experiments. To the best
of our knowledge, this work is the first to report this approach
to the synthesis of GVL directly from EL using supercritical
ethanol as a the hydrogen donor.
a
Table 3 Effect of catalyst loading on the conversion of EL into GVL
Entry
Catalyst loading (g)
GVL yield (%)
EL conversion (%)
1
2
3
4
5
6
0.0
0.3
0.5
0.8
1.0
1.2
0.8
37.7
46.2
56.9
62.5
62.9
1.7
46.0
54.6
71.8
81.5
82.3
Fig. 8 Effect of extra additives on the conversion of EL into GVL. Reaction
2
conditions: EL, 2 g; ZrO -573, 1 g; ethanol, 38 g; reaction temperature, 523 K;
a
Reaction conditions: EL, 2 g; catalyst, ZrO
2
-573; ethanol, 38 g;
reaction time, 1 h. N.O. means that no additives were introduced to the reaction
system.
reaction temperature, 523 K; reaction time, 1 h.
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GC analysis
GVL and EL in the reaction mixture were analyzed on an
Agilent 7890 series equipped with a HP-5 capillary column
(
(
30.0 m 6 320 mm 6 0.25 mm) and a flame ionization detector
FID) operating at 543 K. The carrier gas was N with a flow
2
2
1
rate of 1.0 mL min . The following programmed temperature
2
1
was used in the analysis: 313 K (4 min)–15 K min –623 K (5
min). The yield of GVL and the conversion of EL were
calculated using the following equations:
ꢀ
ꢁ
Scheme 3 Proposed mechanism for the conversion of EL into GVL over ZrO
supercritical ethanol via CTH.
2
in
Mole of EL
EL conversion (%)~ 1{
|100% (1)
Initial mole of EL
Mole of EL
calcined at 573 K, 773 K and 973 K for 12, 6 and 6 h,
GVL yield (%)~
|100%
(2)
Initial mole of EL
respectively. The prepared catalysts were labeled as ZrO
ZrO -773 and ZrO -973, respectively.
2
-573,
2
2
Catalyst characterization
SEM images were performed on a Hitachi S-4800 by using 20
kV or 30 kV. X-ray diffraction (XRD) patterns were obtained on
a Panalytical X’pert Pro diffractometer using a Cu-Ka radiation
source with the following parameters: 40 kV, 30 mA, 2h from 5u
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (2010CB732201), the National
Natural Science Foundation of China (21106121), the
Provincial R&D Program from Economic and Trade
Committee of Fujian Province of China (1270-K42004), the
Fundamental Research Funds for the Central Universities
(2010121077) and the Fundamental Research Funds for the
Xiamen University (201212G006).
2
1
to 80u at a scanning speed of 3u min . FT-IR spectra were
recorded on a Nicolet 330 spectrometer. Thermal gravimetric
analysis and differential thermal gravimetric analysis (TGA/
DTG) were carried out on a SDT Q600 thermal analyzer under a
2
1
2
dynamic N atmosphere (100 mL min ) in the temperature
2
1
range of 293–1173 K with a heating rate of 20 K min , each
sample was held isothermally at 373 K for 30 min before
ramping up to 1173 K. BET surface areas were measured on a
TriStar 3000 surface area and porosimetry analyzer
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This journal is ß The Royal Society of Chemistry 2013