Zirconium Oxide Supported Palladium Nanoparticles as a Highly Efficient Catalyst in the Hydrogenation–Amination of Levulinic Acid to Pyrrolidones

Article abstract of DOI:10.1002/cctc.201600739

The selective hydrogenation–amination of levulinic acid into pyrrolidones is regarded as one of the most important reactions to transform biomass-derived lignocellulose feedstocks into valuable chemicals. Here we report on ZrO₂-supported Pd nanoparticles as a highly active, chemoselective, and reusable catalyst for the hydrogenation–amination of levulinic acid with H₂ and various amines under mild reaction conditions. The Pd/ZrO₂ catalyst exhibited a marked increase in activity compared with conventional Pd catalysts and a significant enhancement in pyrrolidone selectivity. The excellent catalytic performances are reasonably attributed to the ZrO₂ support, which has strong Lewis acidity to enhance the hydrogenation–amination reaction and hinder side reactions.

Full text of DOI:10.1002/cctc.201600739

DOI: 10.1002/cctc.201600739
Full Papers
Zirconium Oxide Supported Palladium Nanoparticles as
a Highly Efficient Catalyst in the Hydrogenation–
Amination of Levulinic Acid to Pyrrolidones
Jian Zhang+,[a] Bin Xie+,[a, d] Liang Wang,*[a] Xianfeng Yi,[b] Chengtao Wang,[a]
Guoxiong Wang,[a] Zhifeng Dai,[a] Anmin Zheng,*[b] and Feng-Shou Xiao*[a, c]
The selective hydrogenation–amination of levulinic acid into
pyrrolidones is regarded as one of the most important reac-
tions to transform biomass-derived lignocellulose feedstocks
into valuable chemicals. Here we report on ZrO2-supported Pd
nanoparticles as a highly active, chemoselective, and reusable
catalyst for the hydrogenation–amination of levulinic acid with
H2 and various amines under mild reaction conditions. The
Pd/ZrO2 catalyst exhibited a marked increase in activity com-
pared with conventional Pd catalysts and a significant en-
hancement in pyrrolidone selectivity. The excellent catalytic
performances are reasonably attributed to the ZrO2 support,
which has strong Lewis acidity to enhance the hydrogenation–
amination reaction and hinder side reactions.
Introduction
The selective transformation of renewable biomass-derived
feedstocks is very important for the sustainable production of
fuels and fine chemicals.[1,2] In particular, levulinic acid (LA),
which could be obtained easily from lignocelluloses in an envi-
ronmentally friendly way, has been recognized as one of the
top 10 most promising platform molecules derived from bio-
mass by the U.S. Department of Energy.[3,4] Recently, many suc-
cessful routes have been developed for the efficient transfor-
mation of LA to valuable chemicals.[5–44] Particularly, the hydro-
genation–amination of LA to pyrrolidones has attracted much
attention because pyrrolidones are used widely as solvents
and raw materials for the production of surfactants and
fibers.[5,16–19] To date, various metal complex catalysts have
been used to catalyze the LA-to-pyrrolidone processes,[16,18] but
their homogeneous nature makes catalyst separation and re-
generation from the reaction system difficult. Supported Pt
and Au metal nanoparticles are regarded as active and recycla-
ble catalysts, but most reactions with these heterogeneous cat-
alysts are performed at a high reaction temperature and/or
with solvents,[5,18,19] which limit their wide applications because
of the high energy cost in the reaction process and product
separation from the solvents. Recently, a molybdenum oxide
modified Pt catalyst was reported for LA hydrogenation–ami-
nation under mild reaction conditions.[17]
[a] J. Zhang,+ Dr. B. Xie,+ Dr. L. Wang, C. Wang, G. Wang, Z. Dai, Prof. F.-S. Xiao
Key Laboratory of Applied Chemistry of Zhejiang Province and Department
of Chemistry
Zhejiang University
Supported Pd nanoparticles are well known as highly active
catalysts for industrial hydrogenation[45] but their use is chal-
lenging in the hydrogenation–amination of LA because of
their disadvantages such as poor pyrrolidone selectivity.[17] If
we consider that Pd catalysts, which have lower costs than Au
and Pt catalysts, have been used widely in industrial processes,
the development of highly efficient Pd catalysts for the hydro-
genation–amination of LA to pyrrolidones is important for sus-
tainable chemical synthesis from biomass.
Hangzhou 310028 (P.R. China)
E-mail: liangwang@zju.edu.cn
fsxiao@zju.edu.cn
[b] X. Yi, Prof. A. Zheng
State Key Laboratory of Magnetic Resonance and Atomic and Molecular
Physics and Mathematics
Wuhan Institute of Physics and Mathematics
Chinese Academy of Science
Wuhan 430071 (P.R. China)
E-mail: zhenganm@wipm.ac.cn
[c] Prof. F.-S. Xiao
In this work, we report for the first time a heterogeneous Pd
nanocatalyst that can efficiently catalyze the hydrogenation–
amination of LA to pyrrolidones. We employed ZrO2 as a sup-
port, and the Pd/ZrO2 catalyst exhibits a superior catalytic ac-
tivity and pyrrolidone selectivity at a very mild reaction tem-
perature (908C) and solvent-free conditions. For example, in
the hydrogenation–amination of LA with n-octylamine, the
Pd/ZrO2 catalyst gives a yield of pyrrolidone of over 98% to
afford a turnover number (TON) of 5625, which is 2–40 times
that of catalysts reported previously.[5a,16–19] Importantly, the
Key Laboratory of Biomass Chemical Engineering of Ministry of Education
Zhejiang University
Hangzhou 310027 (P.R. China)
[d] Dr. B. Xie+
Petrochina Petrochemical Research Institute
Beijing 102206 (P.R. China)
[+] These authors contributed equally to this work.
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600739.
This manuscript is part of a Special Issue on the “Catalytic Conversion of
Biomass”. A link to the Table of Contents will appear here once the Spe-
cial Issue is assembled.
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Pd/ZrO2 catalyst is stable and recyclable and offers a good op-
portunity for potential applications.
characterized by using TEM, which confirmed the high disper-
sion of the Pd nanoparticles on the support (Figures S3 and
S4).
The catalytic tests began with the hydrogenation–amination
of LA with n-octylamine under 0.5 MPa of H2 at 908C as
a model reaction, in which various side reactions of hydroge-
nation and dehydration might occur during the process
(Scheme 1). The catalytic data in the hydrogenation–amination
of LA over various supported Pd catalysts are presented in
Figure 2. Notably, the catalyst support is very sensitive to the
activity and selectivity. Pd/ZrO2 exhibits a full conversion of LA
with a pyrrolidone (5-methyl-1-octylpyrrolidin-2-one) selectivity
as high as 98.7% (Figure 2A), whereas Pd/Al2O3 shows a LA
conversion of 76.1% and a pyrrolidone selectivity of 87.0%.
Results and Discussion
Pd/ZrO2 was synthesized by impregnation. Typically, ZrO2 was
added to a mixed solution of Na2PdCl4 and urea. After the mix-
ture was stirred at 908C for 4 h, the solid was separated from
the liquor and treated by calcination and reduction. Finally,
Pd/ZrO2 with a Pd loading of 2.1% was obtained. For compari-
son, the Al2O3- and TiO2-supported Pd catalysts were synthe-
sized by the same method and were denoted as Pd/Al2O3 and
Pd/TiO2 with Pd loadings of 2.1 and 1.9%, respectively.
A TEM image of Pd/ZrO2 is presented in Figure 1, which
shows direct evidence that the Pd nanoparticles are highly dis-
Scheme 1. Catalytic hydrogenation–amination of LA and the possible side
reactions.
Figure 1. A) TEM image of Pd/ZrO2 and B) HRTEM image of a Pd nanoparti-
cle.
persed on the ZrO2 support. Furthermore, the high-resolution
transmission electron microscopy (HRTEM) image shows that
the Pd nanoparticles have well-defined lattice spacing of
0.223 nm, which is associated with the Pd{111} plane.[46–48] The
Pd nanoparticle size distribution was estimated to be in the
range of 3–7 nm from more than 80 nanoparticles (Figure S1).
XRD patterns of Pd/ZrO2, Pd/TiO2, and Pd/Al2O3 are shown in
Figure S2. In these samples it is difficult to observe the peaks
related to Pd nanoparticles, which suggests the high disper-
sion of the Pd nanoparticles. We used CO adsorption tests to
measure the fraction of exposed Pd atoms in these catalysts,
which is denoted as the Pd dispersion degree (Table 1).
Pd/ZrO2 has a dispersion degree of 21.5%, which is very similar
to that of Pd/Al2O3 and a little lower than that of Pd/TiO2. The
Pd nanoparticles on the Al2O3 and TiO2 supports were further
Table 1. Structural parameters of various Pd catalysts.
Sample
SBET
[m2 gꢀ1
Pd loading
[wt%][a]
Pd nanoparticle
size [nm][b]
Dispersion
[%][c]
]
Pd/ZrO2
Pd/Al2O3
Pd/TiO2
<10
<10
42
2.1
2.1
1.9
3–7
2–7
2–4
21.5
22.9
30.1
Figure 2. A) LA conversion (blue) and pyrrolidone selectivity (red) and
B) TON in the hydrogenation–amination of LA with n-octylamine over vari-
ous oxide-supported Pd catalysts. Reaction conditions: 5 mmol of LA,
5 mmol of amine, S/C at 1000, reaction time at 12 h, 908C, and 0.5 MPa of
H2.
[a] Measured by using ICP-OES. [b] Measured by using TEM. [c] Measured
by using CO adsorption.
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Additionally, Pd/SiO2 and Pd/C exhibit an extremely low pyrroli-
done selectivity (<4.5%). Particularly, Au/ZrO2, Pt/C, and
Pt/MoOx/TiO2, which were reported as very efficient catalysts
for the hydrogenation–amination of LA,[5a,17,19] exhibit pyrroli-
done yields of 24.4, 61.8 and 92.1%, respectively (Table S1).
The Pt/ZrO2 exhibits a high pyrrolidone yield of 99.0%
(Table S1), which is similar to that of Pd/ZrO2 (98.7%), but the
lower cost of Pd than Pt suggests that Pd/ZrO2 is more favora-
ble than Pt/ZrO2 for the hydrogenation–amination reaction.
After the calculation of TON, it is clear that the TON of the
Pd/ZrO2 could reach as high as 5625, which is much higher
than that of the other oxide-supported Pd catalysts (222–1077;
Figure 2B). This value is even higher than that of other metal-
based catalysts (Au, Pt, Ir, and Ru, 115–2150; Table S2) reported
previously.[5a,16a,17,18,19a] These results demonstrate the excellent
activity for LA conversion and the high selectivity for the for-
mation of pyrrolidone in hydrogenation–amination over the
Pd/ZrO2 catalyst.
The dependence of LA conversion and product yield on the
reaction time over the Pd/ZrO2, Pd/Al2O3, and Pd/TiO2 catalysts
is shown in Figure 3. If Pd/ZrO2 is used as a catalyst, a conver-
sion of 94% is reached quickly at 6 h (Figure 3A-a) accompa-
nied by the formation of amine (m/e=229.2, GC–MS) and pyr-
rolidone. The amine appears at the start of the reaction
(Table S3) and reaches the highest yield (28.2%) at 3 h. After
almost the full conversion of LA (>99%) at 8 h, the pyrroli-
done is still formed continuously until the yield is higher than
99.0% at 10 h with the complete consumption of amine in the
reaction. This implies that the pyrrolidone product is formed
through an amine intermediate. In addition, there is nearly no
g-valerolactone product in the reaction (Table S3 and Fig-
ure 3A-d), which means that the direct hydrogenation–esterifi-
cation of LA is avoided completely over the Pd/ZrO2 catalyst.
Conversely, if Pd/Al2O3 and Pd/TiO2 were used as catalysts,
g-valerolactone is formed at the start of the reaction and
formed continuously (Table S3, Figure 3B and 3C), which sug-
gests a direct hydrogenation route from LA to g-valerolactone.
Based on these experimental data and the possible mecha-
nism for LA conversion suggested previously,[6,17] it is proposed
that the hydrogenation–amination of LA to pyrrolidone com-
prises steps of (i) amination of LA to form imine, (ii) hydrogena-
tion of imine to amine, and (iii) condensation of amine to pyr-
rolidone (Scheme 2). In these steps, the formation of amine
should be regarded as the rate-limiting step because the order
of the yields of the amine intermediate formed in the reaction
is Pd/ZrO2 >Pd/Al2O3 >Pd/TiO2. To confirm this order, the cata-
lytic activities of the Pd/ZrO2, Pd/Al2O3, and Pd/TiO2 in the ami-
nation of 2-hexanone with aniline were compared, and their
turnover frequencies (TOFs) are presented in Table 2. Clearly,
Pd/ZrO2 (2600 hꢀ1) is much more active than Pd/TiO2 and
Pd/Al2O3 (756 and 1900 hꢀ1), which demonstrates the stronger
ability for amination over Pd/ZrO2 than Pd/Al2O3 and Pd/TiO2.
As a major side reaction, the conversion of LA to g-valerolac-
tone occurs easily through tandem steps of (i) reduction of the
C=O group on LA to CꢀOH and (ii) esterification to form g- va-
lerolactone (Scheme S1), in which the reduction of C=O is
regarded as a key step.[8] Notably, in the reduction of
Figure 3. Dependence of a) LA conversion and selectivity to b) pyrrolidone,
c) amine, and d) g-valerolactone on time in the hydrogenation–amination of
LA with n-octylamine over A) Pd/ZrO2, B) Pd/Al2O3, and C) Pd/TiO2. The de-
crease of the g-valerolactone yield is because of the further hydrogenation
to alcohols.
Scheme 2. Proposed reaction pathways for the hydrogenation–amination of
LA to pyrrolidone.
2-hexanone to 2-hexanol, Pd/ZrO2 exhibits a much lower activi-
ty than Pd/Al2O3 and Pd/TiO2 (entries 1–3 in Table 3). After the
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Table 2. Catalytic activities in the conversion of 2-hexanone and aniline
to the corresponding amine over Pd/ZrO2, Pd/Al2O3, and Pd/TiO2 in the
presence of H2.[a]
[b]
Entry
Catalyst
TOF [hꢀ1
]
1
2
3
Pd/ZrO2
Pd/Al2O3
Pd/TiO2
2600
1900
756
[a] Reaction conditions: 5 mmol of 2-hexanone, 5 mmol of aniline, S/C at
1000, reaction time 15 min, 908C, and 0.5 MPa of H2. [b] The TOFs were
calculated from the conversion of aniline over all the Pd species in the re-
action system.
Table 3. Catalytic activities in hydrogenation of 2-hexanone to 2-hexanol
over Pd/ZrO2, Pd/Al2O3, and Pd/TiO2.[a]
Figure 4. A) 31P NMR spectra of TMP and B) 13C NMR spectra of 2-13C-acetone
adsorbed on a) ZrO2, b) Al2O3, and c) TiO2. The asterisks in the 13C NMR spec-
tra denote spinning side bands.
[b]
Entry
Catalyst
TOF [hꢀ1
]
1
2
3
4[c]
5[c]
6[c]
Pd/ZrO2
Pd/Al2O3
Pd/TiO2
Pd/ZrO2
Pd/Al2O3
Pd/TiO2
762
1140
1136
[d]
magic-angle-spinning (MAS) NMR spectra of 2-13C-acetone ad-
sorbed on ZrO2, Al2O3, and TiO2 are shown in Figure 4B. In
these spectra, the peak at d=224 ppm is caused by the strong
interaction of Lewis acidic sites with acetone, the peak at
d=216 ppm is attributed to the relatively weak interaction of
Lewis acidic sites with acetone, and the peak at d=210 ppm is
assigned to the physical sorption of acetone. The signals at
d=181–167, 109–70, 42, and 34 ppm are related to the prod-
ucts of the bimolecular and trimolecular reactions of 2-13C-ace-
tone. Interestingly, ZrO2 shows a much more intense peak at
d=224 ppm than Al2O3 and TiO2, which indicates that ZrO2 has
much stronger Lewis acidic sites than Al2O3 and TiO2.
–
270
455
[a] Reaction conditions: 5 mmol of 2-hexanone, S/C at 1000, reaction time
15 min, 3 mL of CH2Cl2 as solvent, 908C, and 0.5 MPa of H2. [b] The TOFs
were calculated from the production of 2-hexanol over all the Pd species
in the reaction system. [c] 3 mmol of aniline was added in the mixture.
[d] Undetectable.
addition of aniline in the starting 2-hexanone, Pd/ZrO2 is com-
pletely inactive for C=O reduction, whereas Pd/Al2O3 and
Pd/TiO2 still give TOFs of 270 and 455 hꢀ1, respectively (en-
tries 4–6 in Table 3). These results indicate that Pd/ZrO2 is
highly selective in the tandem amination and hydrogenation
of the C=O group to amine rather than the direct hydrogena-
tion of the C=O group to CꢀOH species, which can explain
why the byproduct g-valerolactone is inhibited with Pd/ZrO2.
It is well known that ZrO2, Al2O3, and TiO2 supports are Lewis
acidic oxides that could react with the C=O group by an
acid–base interaction to influence the reaction pathways in the
conversion of C=O groups in the compounds.[49] To understand
the activities over various catalysts, we compared the NMR
spectra of adsorbed probe molecules, trimethylphosphine
(TMP) and 2-13C-acetone, to characterize their acidic
properties.[50]
The 31P NMR spectra of TMP adsorbed on ZrO2, Al2O3, and
TiO2 are presented in Figure 4A. The main peaks at d=ꢀ45,
ꢀ49, and ꢀ57 ppm could be assigned to the interaction of
TMP with Lewis acid sites. In these spectra, no clear signals at
d=ꢀ2 to ꢀ5 ppm associated with the interaction of TMP with
Brønsted acidic protons could be observed, which indicates
that the Brønsted acidic sites are absent in these oxides. 13C
The temperature-programmed desorption (TPD) profiles of
acetone on ZrO2, Al2O3, and TiO2 are shown in Figure S5. All
three samples exhibit peaks in the low-temperature region of
70.6–72.88C, which is assigned to the desorption of physically
adsorbed acetone. Interestingly, ZrO2 and Al2O3 exhibit addi-
tional peaks in the high-temperature region (>2508C), which
are assigned to acetone adsorbed strongly on the metal oxides
by acid–base interaction. Notably, the desorption temperature
on ZrO2 (309.18C) is higher than that of Al2O3 (272.88C), which
indicates the stronger interaction of acetone with ZrO2 than
with Al2O3.
The IR spectra of acetone adsorbed on ZrO2, Al2O3, and TiO2
were measured to understand the interaction between ace-
tone and the metal oxides (Figure 5). The spectra of TiO2 and
Al2O3 exhibit peaks at n˜ =1724 and 1715 cmꢀ1 assigned to the
C=O bond of acetone. Interestingly, the vibration of the C=O
bond appeared at n˜ =1675 cmꢀ1 in the spectrum of ZrO2,
which is a redshift of 49 and 40 cmꢀ1 from that of TiO2 and
Al2O3, respectively. The redshift is consistent with the stronger
interaction of the basic C=O group with the Lewis acids on
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Figure 6. Recycling tests (LA conversion: blue; pyrrolidone selectivity: red;
initial reaction rate: green) in the hydrogenation–amination of LA with n-oc-
tylamine over Pd/ZrO2. The reaction conditions are the same as those stated
for Figure 2A. The reaction rates were calculated from the LA conversion in
the first 25 min of the reaction.
Figure 5. IR spectra of acetone adsorbed on a) ZrO2, b) Al2O3, and c) TiO2.
ZrO2 than those on TiO2 and Al2O3.[49,51] This phenomenon is in
good agreement with that observed from the acetone-TPD
profiles (Figure S5) and acetone-adsorption 13C NMR spectra
(Figure 4).
nanoparticle size distribution to that of the as-synthesized cat-
alyst, which suggests the good stability of the Pd nanoparticles
on the ZrO2 support.
Possibly, the abundant strong Lewis acid–base interaction in-
creases the electrophilicity of the C=O group in the com-
pounds significantly, which promotes amination for the forma-
tion of amine intermediates and prevents the hydrogenation
of the C=O group to CꢀOH species in the hydrogenation–ami-
nation. These features make the Pd/ZrO2 catalyst highly active
and selective for the hydrogenation–amination of LA to pyrroli-
dones (Scheme 3).
Encouraged by the successful hydrogenation–amination of
LA with n-octylamine, we employed various amines as reac-
tants in the hydrogenation–amination reactions. As expected,
Pd/ZrO2 exhibits very high yields of the corresponding pyrroli-
done products (Table 4). The high activity, superior selectivity,
excellent stability, and general applicability show the potential
of Pd/ZrO2 in hydrogenation–amination reactions in the future.
Recyclability tests in the hydrogenation–amination of LA
with n-octylamine over Pd/ZrO2 are shown in Figure 6. After
the sixth run, Pd/ZrO2 still gives a LA conversion of 98.5% and
a pyrrolidone selectivity of 98.0%. Further observation shows
that the LA conversion (~14.5%) and reaction rates
(~290 molLA molPdꢀ1 hꢀ1) at the beginning of each run (25 min)
are very similar. All of these results indicate the excellent recy-
clability of the Pd/ZrO2 catalyst. Additionally, inductively
coupled plasma optical emission spectroscopy (ICP-OES) shows
that the Pd species is undetectable in the reaction liquor, TEM
images of used Pd/ZrO2 catalyst (Figure S6) show a similar Pd
Table 4. Catalytic activity of Pd/ZrO2 in the hydrogenation–amination of LA
with various amines.[a]
Entry Amine
Product
Conversion Yield
[%][b]
94.4
[%]
1
82.7
2
95.9
90.0
97.5
90.0
80.1
93.6
3[c]
4
[a] Reaction conditions: 5 mmol of LA, 5 mmol of amine, S/C at 1000, reac-
tion time at 12 h, 908C, 0.5 MPa of H2. [b] The conversion of amines. [c] Reac-
tion time at 17 h.
Scheme 3. The role of ZrO2 in two pathways of LA conversion.
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substrate and catalyst were mixed in the reactor. Then the reaction
system was heated quickly to a given temperature (the tempera-
ture was measured with a thermometer in an oil bath). Then hy-
drogen was introduced and kept at desired pressure (the hydrogen
pressure in the autoclave was recorded at the reaction tempera-
ture). After the reaction, the product was diluted and analyzed by
using gas chromatography using a flame ionization detector with
a flexible quartz capillary column coated with FFAP (Free Fatty
Acid Phase). The conversion of LA (CLA), the TON, the yield (Y), and
the selectivity to pyrrolidone (Spyrrolidone) were calculated from Equa-
tions (1)–(4), respectively:
Conclusions
We demonstrate that ZrO2-supported Pd nanoparticles are
highly efficient for the selective production of pyrrolidones
from the hydrogenation–amination of levulinic acid (LA) with
amines and H2. The excellent properties are attributed to the
presence of strong Lewis acidity in the catalyst, which is very
favorable for LA amination and hinders the side reaction of
direct LA hydrogenation–esterification. This work is important
for the development of highly efficient catalysts for the conver-
sion of LA.
CLA ½%ꢁ ¼ ½1ꢀðMLA after reaction=MLA before reactionÞꢁ ꢂ 100 %
TON ¼ ðMLA before reactionꢀMLA after reactionÞ=MPd
ð1Þ
ð2Þ
ð3Þ
ð4Þ
Experimental Section
Ypyrrolidone ½%ꢁ ¼ ðMpyrrolidone=MLA before reactionÞ ꢂ 100 %
Spyrrolidone ½%ꢁ ¼ ðYpyrrolidone=CLAÞ ꢂ 100 %
Materials: All the chemicals are commercially available and used
directly without any treatment.
in which MLA before reaction and MLA after reaction are the molar amount of
LA before and after reaction, respectively. MPd is the molar amount
of total Pd atoms in the reaction system. Mpyrrolidone is the molar
Synthesis of ZrO2 support: ZrOCl2 (3 g) was dissolved in deionized
H2O (500 mL). Then, NaOH aqueous solution (1m) was dropped
into the above solution until white precipitate was fully formed.
After hydrothermal treatment in an autoclave at 2008C overnight,
filtration at RT, washing with a large amount of water, drying at
808C, and calcination at 5008C for 2 h, the ZrO2 support was
obtained.
amount of pyrrolidone product after reaction. MLA
,
before reaction
MLA after reaction, and Mpyrrolidone were determined by using GC with bro-
mobenzene as an internal standard.
The recyclability of the catalyst was tested by separating it from
the reaction system by centrifugation, washing with large quantity
of ethanol/cyclohexane, and drying at 1208C for 2 h.
Loading of Pd on the support: Typically, the solid support (1 g)
was added to Na2PdCl4 solution (50 mL) with the desired Pd con-
centration. After the addition of urea (molar ratio of urea/Pd at
100), stirring at 908C for 4 h in a closed reactor, filtration at RT,
washing with a large amount of water, drying at 1008C for 12 h,
calcination at 4008C for 3 h, and reduction by H2 at 2508C for 2 h,
the supported Pd catalyst was obtained, which was designed as
Pd/ZrO2. The Pd loading for Pd/ZrO2, Pd/Al2O3, Pd/TiO2, Pd/SiO2,
and Pd/C was 2.1, 2.1, 1.9, 2.0, and 5.0 wt%, respectively. The sup-
ported Pt and Au catalysts were synthesized by the same method
except with the use of H2PtCl6 and HAuCl4 as the metal precursors.
Acknowledgements
This work is supported by the National Natural Science Founda-
tion of China (21333009, U1462202, 21403192, and 21522310).
Keywords: biomass · hydrogenation · palladium · supported
catalysts · zirconium
Characterization: XRD patterns were collected by using a Rigaku
D/MAX 2550 diffractometer with CuKa radiation (l=1.5418 ꢁ). Pd
contents were determined by using ICP-OES (PerkinElmer 3300DV).
TEM was performed by using a JEM-2100F electron microscope
(JEOL, Japan) with an acceleration voltage of 200 kV. NMR spectros-
copy was performed by using a Bruker Ascend-500 spectrometer
at resonance frequencies of 125.88 and 202.63 MHz for 13C and 31P,
respectively. The experiments were recorded using a 4 mm triple-
resonance MAS probe at a spinning rate of 10 kHz. The pulse
width (p/2) for 13C was measured to be 4.3 ms. A contact time of
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Published online on && &&, 0000
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&
These are not the final page numbers! ÞÞ

FULL PAPERS
J. Zhang, B. Xie, L. Wang,* X. Yi, C. Wang,
G. Wang, Z. Dai, A. Zheng,* F.-S. Xiao*
&& – &&
Zirconium Oxide Supported Palladium
Nanoparticles as a Highly Efficient
Catalyst in the Hydrogenation–
Amination of Levulinic Acid to
Pyrrolidones
Selectivity secrets: ZrO2-supported Pd
nanoparticles (Pd/ZrO2) are highly effi-
cient for the selective conversion of
levulinic acid to pyrrolidones by the hy-
drogenation–amination route.
&
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8
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ÝÝ These are not the final page numbers!