Zirconium-cyanuric acid coordination polymer: Highly efficient catalyst for conversion of levulinic acid to γ-valerolactone

Article abstract of DOI:10.1039/c5cy02215k

Transformation of levulinic acid (LA) to γ-valerolactone (GVL) is essential for biomass conversion into value-added chemicals. The development of an efficient and heterogeneous catalyst for the above reaction has become a hot topic. In this work, a porous zirconium-cyanuric acid coordination polymer (Zr-CA) with an average pore diameter of 3.5 nm was successfully synthesized. As expected, the as-synthesized Zr-CA has very high activity for the catalytic transformation of LA and its esters to GVL. Systematic examinations indicated that both the acidic sites and the basic sites contributed significantly to the excellent catalytic performance of Zr-CA, which verified our assumption. This work provides a new route for designing cyanuric acid-based solid acid catalysts.

Full text of DOI:10.1039/c5cy02215k

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DOI: 10.1039/C5CY02215K
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Zirconium-cyanuric acid coordination polymer: Highly efficient
catalyst for conversion of levulinic acid to γ-valerolactone
Zhimin Xue,a* Jingyun Jiang,b Guofeng Li,b Wancheng Zhao,b Jinfang Wangb and Tiancheng Mub*
twoReceived 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Transformation of levulinic acid (LA) to γ-valerolactone (GVL) is essential for biomass conversion to value-added chemicals.
Development of efficient and heterogeneous for the above reaction has become a hot topic. In this work, porous
zirconium-cyanuric acid coordination polymer (Zr-CA) with average pore diameter of 3.5 nm was successfully synthesized.
As expected, the as-synthesized Zr-CA has very high activity for catalytic transformation of LA and its esters to GVL.
Systematic examinations indicated that both the acidic sites and the basic sites contributed significantly to the excellent
catalytic performance of Zr-CA, which verified our assumption. This work provides a new route for designing cyanuric acid
based solid acid catalytic catalysts.
As a well-known reaction, transfer hydrogenation (TH)
reaction is an attractive alternative to H2 for the reduction of
carbonyl compounds.9 In the process for converting LA and its
Introduction
esters into GVL, the reduction of carbonyl groups is the key
step. It has been reported that formic acid could be used as
the hydrogen source for the TH of LA to produce GVL in the
presence of Au, Ru and Ag catalysts.10 However, the drawbacks
of the use of noble metals and the corrodibility of formic acid
have limited the application of formic acid for the TH of LA.
Compared with formic acid, using alcohols as the hydrogen
source for the TH of LA and its esters into GVL has attracted
increasing interests in recent years. Up to now, several
catalytic systems have been developed for the TH reactions of
LA and esters to GVL using various alcohols as the hydrogen
source, including homogeneous Ru complexes,11 RANEY® Ni,12
ZrO2,13 Zr(OH)4,14 Zr-beta zeolites,15 Zr-HBA,16 and porous
zirconium-phytic acid hybrid.17 However, some drawbacks are
still to be overcome, including harsh reaction temperatures,
long reaction time, low catalyst reactivity or stability, etc.
Overall, the preparation of efficient, inexpensive, and easily
recycled catalysts for GVL production by the TH reactions is
only at the beginning stage and urgently requires further
developments.
Transformation of biomass and its derivatives into liquid fuels
and value-added chemicals has been recognized as an
alternative way for fossil-fuel resources, and has received
much attention in the past decades.1 Many efficient strategies
have been established for conversion of renewable biomass
into various value-added chemicals,2 such as various alcohols3
and organic acids,4 furan-based compounds,5 and γ-
valerolactone (GVL),6 etc. Among these biomass-based
chemicals, GVL has been identified as a versatile platform
chemical for many applications, such as the precursor for
valuable chemicals, a fuel additive, and a bio-based green
solvent.7 Currently, GVL is mainly produced by the metal (e.g.
Ru, Pd, Rh, Co, Ni, and Cu) catalytic hydrogenation of levulinic
acid (LA) and its esters,8 which could be derived from cellulose
by a simply hydrolysis process. Although good yields of GVL
could be achieved, these catalytic systems suffer from the use
of noble metals, high H2 pressure, and low catalyst stability or
reactivity, etc. Therefore, development of new efficient
methods for the production of GVL is a highly attractive task.
Generally, the increase of the catalyst basicity could
enhance the catalytic activity for the TH reaction mentioned
above.13a,17 Introducing more basic sites to increase the
basicity of the catalysts is an attractive way to design efficient
catalysts for the TH reaction of LA and its esters to produce
GVL. Owing to the ability to coordinate with metal ions,
cyanuric acid (CA, Scheme S1) could be used to prepare metal-
ligand coordination polymers. More importantly, the existence
of phenate groups and pyridinic type N atoms in the structure
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of CA, which are both basic groups, provides the possibility to Catalytic activity of various catalysts for the TH reaction of EL
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DOI: 10.1039/C5CY02215K
design efficient catalysts with higher basicity by using CA as
the building block, which has not been reported. Herein, we
designed the zirconium-CA coordination polymer (Zr-CA) as
efficient heterogeneous catalyst for the TH reaction of LA and
its esters to produce GVL. To the best of our knowledge, this is
the first work for the synthesis of Zr-CA coordination polymer
and its applications to catalyze the TH reaction for GVL
production.
In the initial investigation, ethyl levulinate (EL) was selected as
the model reactant to study the catalytic activities of various
catalysts for the production of GVL through TH reactions
(Table 1). It is obvious that there was no reaction happened in
the absence of any catalyst (Table 1, entry 1). Among the
examined catalysts, the prepared Zr-CA catalyst showed the
o
best performance with the GVL yield of 82.1% at 130 C with a
reaction time of 4 h (Table 1, entry 2). In comparison, Zn-CA
(Table 1, entry 3) and Sn-CA (Table 1, entry 4) showed very low
activity for the TH reaction of EL under the same reaction
conditions. There were two possible reasons for the poor
catalytic activity of Sn-CA and Zn-CA compared with Zr-CA.
Firstly, as detected by NH3-TPD and CO2-TPD (Fig. S4), the
acidity and basicity of Sn-CA and Zn-CA were much weaker
than Zr-CA, which could result in the poor activity of Sn-CA and
Zn-CA. Secondly, the nature of different metal also affected
their catalytic activity and Zr showed the superior activity for
the TH reaction compared with Sn and Zn.16, 17 Meanwhile,
when CA alone was used as the catalyst, there was no GVL
produced (Table 1, entry 6). In addition, silica gel, which
possessed large amounts of hydroxyls, was used to catalyze
the TH reaction (Table 1, entry 6). However, there was nearly
no reaction happened, indicating that the Brønsted acidity of
the hydroxyls had no catalytic activity for the TH reaction.
Generally, in the TH reaction of LA and its esters, Lewis acidic
sites were the main active center, which will be dicussed
below. Furthermore, it was found that Zr-CA prepared in this
work showed higher activity than most of other usual Zr-
containing catalysts reported in the literature under
comparable reaction conditions. These results in Table 1
indicated that Zr-CA was the highly efficient catalyst for the
GVL production from the TH reaction of EL.
Results and discussion
Catalyst preparation and characterization
The synthetic procedure for Zr-CA was presented in detail in
the Experimental section. The synthesized Zr-CA was firstly
characterized by X-ray diffraction (XRD), transmission electron
microscopy (TEM), scanning electron microscopy (SEM), N2
adsorption-desorption examination, and Fourier transform
infrared spectra (FT-IR). The XRD pattern showed one broad
diffraction peak, suggesting that the obtained Zr-CA was
amorphous (Fig. 1a). Meanwhile, SEM and TEM examinations
demonstrated that Zr-CA had no uniform shape (Fig. 1b and
1c). Furthermore, Zr-CA was porous detected by N2
adsorption-desorption isotherm (Fig. 1d), and the surface area,
pore volume, and average pore diameter were 108 m2g-1, 0.13
cm3g-1, and 3.5 nm, respectively (Table S1). In addition, FT-IR
spectra of Zr-CA displayed the characteristic asymmetric (1556
cm-1) and symmetric vibration (1448 cm-1) of phenate groups
in deprotonated CA, which were narrowed compared with the
spectrum of CA (Fig. S1), indicating that the phenate groups of
CA were coordinated to Zr4+ ions.18 The porous structure and
low crystallinity indicated that Zr-CA may be used as a good
catalyst. For comparison, other metal (Sn and Zn)-CA catalysts
were also prepared with similar textural and structural
properties (Figs. S2-S3 and Table S1).
Table 1. TH reaction of EL to produce GVL catalyzed by various
catalysts.a
O
O
O
Catalyst
T, t
OH
OEt
O
+
+
O
EL
GVL
Entry
Catalyst
T (oC)b
t (h)b
C (%)b
GVL Y
(%)b
0
82.1
0.2
4.3
2.4
0.1
0
97.1
84.7
98.2
88.5
98.9
92
S (%)c
1
2
3
4
5
None
Zr-CA
Zn-CA
Sn-CA
ZrO2
130
130
130
130
130
130
130
150
150
200
200
150
150
150
150
4
4
4
4
4
4
4
4
16
1
0
89.3
0.7
8.6
3.4
--
91.9
28.6
50.0
70.6
--
6
7
CA
7.3
1.2
Silica gel
Zr-CA
ZrO2
Zr-CA
Zr(OH)4
Zr-CA
Zr-Beta
Zr-CA
Zr-PhyA
--
8d
9d,e
10
11f
12g
13g,h
14
15i
100
100
100
93.6
100
100
100
100
97.1
84.7
98.2
88.5
98.9
92
1
2
6
4
Fig. 1. Characterization of the Zr-CA. Powder XRD pattern (a), SEM
image (b), TEM image (c), and N2 adsorption-desorption isotherm
(d).
96.9
96.7
96.9
96.7
6
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aReaction conditions: a stainless reactor of 15 mL, 0.2 g catalyst, 1 mmol with a GVL yield of 86.7% due to the steric hindrance of butyl
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EL, 6 g isopropanol. bT=Temperature, t=Time, C=Conversion, Y=Yield, (Table 2, entry 3). Compared with esters, LA could be fully
DOI: 10.1039/C5CY02215K
o
and S=Selectivity. Conversion and GVL yield were determined by GC. transformed into GVL with a yield of 96.8% within 4 h at 130 C
cThe other products were mainly isopropyl levulinate by (Table 2, entry 4) due to the self-acidity of LA, which could
transesterification. dThe hydrogen donor was 2-butanol. eData was improve the lactonization of 4-hydroxypentanoate (shown in
f
obtained from reference 13a. Data was obtained from reference 14. Scheme 1) and thus increase the reaction rate. These findings
i
gThe reactant was LA. hData was obtained from reference 15b. Data further proved that Zr-PhyA showed excellent activity for the
was obtained from reference 17.
TH reaction of LA and its esters to produce GVL.
Table 2. Conversion of LA and its esters into GVL catalyzed by
Influence of other reaction parameters on the reaction
Zr-CA.a
t (h)b
C (%)b
GVL Y
(%)b
S (%)b
We studied the influence of other parameters on the reaction.
It was found that the activity of Zr-CA increased with the rising
of reaction temperatures (Fig. 2). As shown in the figure, when
Entry
Substrate
1
2
3
4
Methyl levulinate
Ethyl levulinate
Butyl levulinate
Levulinic acid
10
10
10
4
100
99.8
91.6
100
97.2
97.1
86.7
96.8
97.2
97.3
94.6
96.8
o
the reaction was conducted at 150 C, the reaction could be
completed in 4 h with a GVL yield of 96.9%. In contrast, 10 h
was needed to fully convert EL into GVL with a yield of 97.1%
o
aReaction conditions: a stainless reactor of 15 mL; 0.2 g Zr-CA; 1 mmol
at 130 C. Meanwhile, the conversion of EL was 81.7% with a
o
reactant; 6 g isopropanol; reaction temperature 130 C. bC=Conversion,
o
GVL yield of only 73.3% at 110 C with a reaction time of 10 h.
Furthermore, the amount of Zr-CA used could also affect the
efficiency of the reaction significantly. The results in Fig. S5
indicated that the conversion of EL and the yield and
selectivity of GVL increased along with the amount of the
catalyst. Generally, higher isopropanol/EL ratio was favourable
for the equilibrium of hydrogen transfer between isopropanol
and EL, and thus enhanced the TH of EL to GVL.19 The influence
of the isopropanol/EL mole ratio was also examined (Table S2).
It was found that the conversion of EL increased with the
isopropanol/EL mole ratio from 1:1 to 17:1 and then remained
nearly unchanged. In contrast, the yield and selectivity of GVL
constantly increased. Under lower isopropanol/EL ratio, the
transesterification of isopropanol and EL was increased and
minute by-products from aldol condensation between EL and
t=Time, Y=Yield, and S=Selectivity. The other products were mainly
isopropyl levulinate obtained by transesterification or esterification.
Reusability of Zr-CA
The reusability of Zr-CA was also examined. It was found that
Zr-CA could be reused at least five cycles without reducing the
conversion of EL and the yield of GVL, as shown in Fig. 3. The
recovered Zr-CA was characterized by SEM, TEM, XRD, and FT-
IR (Fig. S6), which indicated that the properties of Zr-CA were
not changed notably after being used. Meanwhile, the
decomposition temperature of the Zr-CA was about 260 oC
detected by the thermogravimetric (Fig. S7) analysis, which
was much higher than the reaction temperature. Additionally,
N2 adsorption-desorption (Table S1) and XPS (Table S3)
examinations proved that the surface characteristics and
textural properties of fresh and five-times used Zr-CA
remained almost unchanged. Furthermore, no further reaction
was detected after removing Zr-CA from the reaction mixture
in-situ generated acetone were also found with the
14
isopropanol/EL ratio from 1:1 to 33:1.13a,
Therefore, we
conducted the reaction with a isopropanol/EL mole ratio of
100:1.
o
when the reaction was conducted at 130 C for 2 h (Fig. S8).
This result confirmed that Zr-CA was heterogeneous in the
reaction process. At the same time, the concentration of Zr
was very low (<1 ppm) in the reaction mixture detected by ICP,
indicating that the leaching of Zr into the liquid phase was
negligible during the reaction.
Fig. 2. Influence of reaction temperature on the EL conversion
(a) and GVL yield (b). Reaction conditions: a stainless reactor of
15 mL; 0.2 g Zr-CA; 1 mmol EL; 6 g isopropyl alcohol.
Catalytic performance of Zr-CA for various substrates
We also studied the catalytic performance of the prepared Zr-
CA for various substrates. As shown in Table 2, it was found
that Zr-CA could also catalyze the TH reaction of LA and other
esters to produce GVL. Methyl levulinate had a high activity
like EL to give a GVL yield of 97.2% (Table 2, entry 1). However,
butyl levulinate showed lower reaction activity than ML and EL
Fig. 3. The reusability of Zr-CA at 130 oC with reaction times of
4 h (a) and 10 h (b). Reaction conditions: a stainless reactor of
15 mL; 0.2 g Zr-CA; 1 mmol EL; 6 g isopropanol. Black bar for
conversion of EL, red bar for yield of GVL, and blue bar for
selectivity of GVL.
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Explanation of the higher catalytic activity of Zr-CA
View Article Online
DOI: 10.1039/C5CY02215K
It has been reported that the acidity and basicity affected the
activity of the catalysts for the TH reaction of carbonyl
compounds. As shown in Table 1, Zr-CA showed a much better
performance than the common catalyst ZrO2, which was
caused by the higher Lewis acidity and basicity of Zr-CA.
Fig. 4. NH3-TPD spectra (a), XPS spectra of Zr 3d (b), and CO2-
Firstly, we examined the acidity of Zr-CA and ZrO2 by NH3-
TPD method (Fig. 4a). It has been reported that the amount
and strength of Lewis and Brønsted acid sites can be measured
by NH3-TPD method because NH3 as a smaller molecule than
TPD spectra of Zr-CA and ZrO2.
Mechanism
pyridine can interact with most of acidic protons, electron As discussed above, acidic and basic sites in the Zr-CA played
acceptor sites, and hydrogen from neutral or weakly acidic very important role on the TH reaction of LA and its esters.
hydroxyls.20 As shown in Fig. 4a, Zr-CA showed a peak at about NH3-TPD (Fig. 4a) and CO2-TPD (Fig. 4c) examinations
o
260 C, which was related to weak acid site. However, ZrO2 suggested that there were large amounts of acid sites
showed no obvious peaks. At the same time, the amount of (originating from Zr4+) and basic sites (resulting from O2- in
NH3 desorbed followed the order: Zr-CA>ZrO2, which was phenate groups and pyridinic type N atoms) in Zr-CA. In
consistent with their catalytic activity. Because there were addition, we found that the conversion of isopropanol to
nearly no hydroxyls in Zr-CA, we considered that the acid sites acetone/hydrogen did not happen in the absence of LA and its
were mainly resulted from the Zr4+. Generally speaking, the esters, indicating that LA and its esters played an important
metal ions in metal-ligand coordination polymers showed role of the active hydrogen accepter. Based on the
Lewis acidity.21 Based on the discussion above, we assumed experimental results and some related knowledge in the
that the acidic sites in Zr-CA and ZrO2 were mainly Lewis acidic literature,13-17 a possible mechanism for the TH reaction of LA
sites, and the Lewis acidity followed the order: Zr-CA>ZrO2. XPS and its ester to produce GVL over Zr-CA was proposed
examinations showed that the binding energies of Zr in Zr-CA (Scheme 1). In the catalytic cycle, the acidic-basic sites on Zr-
were higher than in ZrO2 (Fig. 4b), which indicated the higher CA resulted in the dissociation of isopropyl alcohol to the
positive charge on the Zr atoms in Zr-CA and thus resulted in a corresponding alkoxide and hydrogen. Meanwhile, Zr4+ in Zr-
stronger Lewis acidity.17,22 Both NH3-TPD and XPS methods CA activated the carbonyl group in LA and its esters. Then,
proved that Zr-CA possessed higher Lewis acidity than ZrO2, hydrogen transfer between the dissociated alcohol and the
which was helpful for the activation of the carbonyl groups in activated LA or ester levulinate occurred through a six-link
EL and thus improved the reaction activity.17
intermediate to form 4-hydroxypentanoate or its ester. Finally,
Secondly, CO2-TPD method suggested that Zr-CA had GVL is obtained from 4-hydroxypentanoate through
much stronger basicity than ZrO2 (Fig. 4c). In Zr-CA, there were intramolecular esterification or transesterification.
many phenate groups, which had been reported that phenate
groups could increase the basicity of the catalyst.16 Meanwhile,
the pyridinic type N atoms in CA could also increase the
basicity of Zr-CA. Therefore, the phenate groups and pyridinic
type N atoms resulted in the higher basicity of Zr-CA than ZrO2.
The higher basicity was beneficial to the dissociation of the
hydroxyl groups in isopropyl alcohol, and thus increasing the
reaction activity of the TH reaction of EL.13a,17
Except for the acidity and basicity of the catalysts, the
textural properties could also affect the catalytic activity of Zr-
CA and ZrO2. As detected by N2 adsorption-desorption, the
surface area, pore volume and average pore diameter of ZrO2
were 21 m2g-1, 0.01 cm3g-1 and 0.53 nm, respectively (Table S1).
It was obvious that Zr-CA had a higher BET surface area, larger
pore diameter, and larger pore volume than ZrO2 (Table S1),
Scheme 1. Plausible reaction mechanism for TH reaction of LA
and its esters into GVL catalyzed by Zr-CA.
which was helpful to increase the diffusion of reactants to the
activity center (zirconium) in Zr-CA, and thus promoted the
reaction with a higher activity.
Conclusions
In summary,
a
new porous zirconium-cyanuric acid
coordination polymer, Zr-CA, has been designed by the
reaction of ZrCl4 and cyanuric acid and used as the
heterogeneous catalyst for the TH reaction of LA and its esters
4 | J. Name., 2012, 00, 1-3
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Moles of L formed
DOI: 10.1039/C5CY02215K
Moles of LA or esters used
to produce GVL with isopropanol as the hydrogen resource. Zr-
CA showed very high performance for the TH reaction with
good GVL yields obtained under the optimal reaction
conditions. Detail examinations indicated that both the acidic
sites and the basic sites contributed significantly to the
excellent catalytic performance of Zr-CA for the TH reaction of
LA and its esters. In addition, the catalyst can be reused at
least five times without reducing the activity and selectivity.
We believed that that the prepared Zr-CA catalyst has great
potential of application in TH reaction of LA and its esters, and
that CA can be used to design other effective metal-CA
coordination polymers.
View Article Online
ield
Moles of L formed
Moles of LA or esters converted
Selectivit
Reusability of Zr-CA
To examine the reusability of the prepared Zr-CA, the used Zr-
CA was recovered by centrifugation, and washed with ethyl
ether (5×10 mL). After drying under vacuum at 80 °C for 24 h,
the catalyst was reused directly for the next run.
Acknowledgements
The authors thank the Beijing Forestry University Fundamental
Research Funds for the Central Universities BLX2014-40 and
National Natural Science Foundation of China (21503016,
21473252) for financial support.
Experimental
Materials
N,N-Dimethylformamide
(DMF),
triethylamine,
ZrO2,
SnCl4·6H2O, ethanol, Zn(NO3)2·6H2O were A. R. grade and were
purchased from Sinopharm Chemical Reagent Beijing Co., Ltd.
Cyanuric acid (CA) with C. P. grade was received from
Sinopharm Chemical Reagent Beijing Co., Ltd. ZrCl4 (98%),
levulinic acid (LA, 99%), ethyl levulinate (EL, 98%), and methyl
levulinate (98%) were obtained from J&K Scientific Ltd. Butyl
levulinate was purchased from TCI (Shanghai) Development
Co., Ltd.
Notes and references
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Synthesis of Zr-CA
3
4
5
ZrCl4 (30 mmol) and cyanuric acid (CA, 40 mmol) were
dissolved in DMF (800 mL) in a flask of 1000 mL. After
dissolved completely, triethylamine (300 mmol) was added
into the solution dropwisely in 2 h. The mixture was firstly
stirred for 10 h at room temperature and then aged under
static conditions at 80 C for 2 h. Finally, the white precipitate
was separated by filtration, thoroughly washed with DMF,
ethanol and ethyl ether and dried at 80 C under vacuum for
24 h. The yield of niobium phytic was about 95% (7.52g).
Found: 36.7% Zr, 22.1% N, 19.2% C. Calcd. for ZrC4N4O4
(259.29): 35.2% Zr, 21.6% N, 18.5% C. Meanwhile, other
catalysts with Sn4+ and Zn2+ ions were synthesized using a
similar route for Zr-CA.
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5,
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,
,
Transfer hydrogenation of LA and its esters
Luterbacher, J. M. Rand, D. M. Alonso, J. Han, J. T.
Youngquist, C. T. Maravelias, B. F. Pfleger and J. A. Dumesic,
Science, 2014, 343, 277-280.
In a typical experiment, reactant (LA or its esters, 1 mmol),
isopropanol (6 g) and the catalyst (0.2 g) were charged into a
stainless reactor of 15 mL. Under the pressure generated
autogenously during the course of reaction, the reaction
mixture was stirred at a known temperature for the desired
time. The products were analyzed quantitatively by gas
chromatography (GC, Agilent 6820) using ethylbenzene as the
internal standard, and identification of the products was done
by GC-MS (Shimadzu QP2010). The conversion of LA or esters
and the yield and selectivity of GVL were calculated using the
following equations:
8
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Moles of LA or esters converted
Conversion
Moles of LA or esters used
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Catalysis Science & Technology
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Catalysis Science & Technology
View Article Online
DOI: 10.1039/C5CY02215K
Graphical abstract
Zirconiumꢀcyanuric acid coordination polymer has been designed and used as
heterogeneous catalyst for efficient catalytic conversion of levulinate acid and esters
to γꢀvalerolactone.