Hydrogenation of 2-Ethylhexenal Using Supported-Metal Catalysts for Production of 2-Ethylhexanol

Article abstract of DOI:10.1007/s10562-016-1940-1

Abstract: The catalytic hydrogenation of 2-ethylhexenal was investigated over Pd supported on ZrO₂, CeO₂, Al₂O₃, MCM-41, MAS-7 and SBA-15. The activities and the selectivities of the catalysts were strongly affected by the nature of the support. Pd/ZrO₂ had an excellent catalytic performance for the hydrogenation. The superior dispersion of Pd on the support ZrO₂, and the stable structure of active components on ZrO₂ as well as the synergistic effect of the bifunctional metal-support interaction enhanced the catalytic performance of Pd/ZrO₂. The conversion of 2-ethylhexenal and the selectivity for 2-ethylhexanol were 100 and 99.1% respectively when the reaction was carried out at 240 °C for 7?h. The product was easily separated from the catalyst and the catalyst was of good reusability when it was repeated six times. In addition, the aggregation of Pd nanoparticles and the coking of ZrO₂ the catalysts were the main cause for the catalyst deactivation. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1940-1

Catal Lett (2017) 147:987–995
DOI 10.1007/s10562-016-1940-1
Hydrogenation of 2-Ethylhexenal Using Supported-Metal
Catalysts for Production of 2-Ethylhexanol
Guoxiu Liu1 · Shiwei Liu1 · Siyuan Liu1 · Shitao Yu1 · Lu Li1 · Fusheng Liu1 ·
Congxia Xie1 · Xiuyan Song1
Received: 11 August 2016 / Accepted: 28 November 2016 / Published online: 28 February 2017
© Springer Science+Business Media New York 2016
Abstract The catalytic hydrogenation of 2-ethylhexenal
was investigated over Pd supported on ZrO2, CeO2, Al2O3,
MCM-41, MAS-7 and SBA-15. The activities and the
selectivities of the catalysts were strongly affected by the
nature of the support. Pd/ZrO2 had an excellent catalytic
performance for the hydrogenation. The superior disper-
sion of Pd on the support ZrO2, and the stable structure of
active components on ZrO2 as well as the synergistic effect
of the bifunctional metal-support interaction enhanced the
catalytic performance of Pd/ZrO2. The conversion of 2-eth-
ylhexenal and the selectivity for 2-ethylhexanol were 100
and 99.1% respectively when the reaction was carried out
at 240°C for 7 h. The product was easily separated from
the catalyst and the catalyst was of good reusability when
it was repeated six times. In addition, the aggregation of Pd
nanoparticles and the coking of ZrO2 the catalysts were the
main cause for the catalyst deactivation.
Graphical Abstract
Keywords Hydrogenation · Catalysis · 2-ethylhexenal ·
2-ethylhexanol
1 Introduction
2-Ethylhexanol is an important chemical intermediate
and predominately converted to dioctyl adipate and dioc-
tyl phthalate which are excellent and harmless plasticizers
for PVC. It is also extensively used in the product of anti-
oxidants, adhesives, cosmetics, surfactants, etc. Currently
almost all 2-ethylhexanol starts from the hydroformylation
of propene to give n-butanal. N-Butanal is converted to
2-ethylhexenal by a base catalyzed aldol condensation reac-
tion, and then hydrogenated to obtain 2-ethylhexenol [1].
Among the above process, the hydrogenation of 2-ethylhex-
enal is a key link in the synthesis of 2-ethylhexenol, and its
result directly affects the yield and quality of 2-ethylhex-
anol. Usually, the hydrogenation is carried out in the gas
phase over Ni or Cu catalyst [2–6]. This process has some
disadvantages such as high consumption of energy, severe
conditions, and low selectivity to the product. Therefore, it
is necessary to explore a new approach for the hydrogena-
tion of 2-ethylhexanol.
* Shiwei Liu
liushiwei@126.com
* Shitao Yu
yushitaoqust@126.com
1
College of Chemical Engineering and of Chemistry
and Molecular Engineering, Qingdao University of Science
and Technology, No. 53 Zhengzhou Road, Qingdao 266042,
China
Vol.:(0123456789)
1 3

988
G. Liu et al.
The carrier has a great influence on the catalytic perfor-
2 Experimental
2.1 Materials
mance of the catalyst, and the common catalytic carriers are
metal oxide and molecular sieves which generally have high
specific surface area and stable structure, and often used to
support a variety of metal catalysts such as Pd, Ru, Rh, Ni,
Cu and Fe. The obtained supported-metal catalysts can be
used for the hydrogenation reaction [7–9], oxidation reac-
tion [10, 11] and condensation reaction [12, 13] with the
high conversion of the reagent and selectivity of the desired
product. The Ru/C, Ru/SBA, Au/ZrC and Au/ZrO2 cata-
lysts were used to catalyze the hydrogenation of levulinic
acid to γ-valerolactone. Levulinic acid was almost converted
to γ-valerolactone with above 90% yield in water solvent,
and Au/ZrO2 showed excellent activity and recyclability
which gave 97% yield of γ-valerolactone [14]. The catalyst
Ni/Al2O3 modified by Cu decreased hydroformylation and
methylcyclohexane byproducts by, respectively 70 and 10%
the hydrogenation of benzene to cyclohexane [15]. Cu/SBA-
15 was an efficient catalyst for the hydrogenation of ethylene
carbonate to synthesize ethylene glycol and methanol, and
100% conversion of ethylene carbonate, 94.7% yield of ethyl-
ene glycol and 62.3% yield of methanol were achieved [16].
37% selectivity of γ-butyrolactone was obtained over the Ni/
CeO2 catalyst but only 12.5% selectivity on the Ni/Al2O3 for
the maleic anhydride hydrogenation [17]. The above results
show that the carriers can improve the dispersion of the
active ingredient and change the catalytic performance of
the catalyst by the interaction with the metal. Although there
have been some studies on the hydrogenation of 2-ethylhex-
enal in the presence of the supported-metal catalyst, they did
not achieve the desired catalytic performance. U. Shröder
et al. investigated the catalytic performance of Pt/Al2O3 on
gas-phase hydrogenation of the unsaturated aldehyde with the
influence of oxygen, although the addition of oxygen (<0.5%)
prevented deactivation on a degree, the side reaction was also
carry out and the yield of 2-ethylhexanol was low [18]. A
series of supported Ni/γ-Al2O3 samples were also prepared
and used in the aldehyde hydrogenation, while its excellent
activity need relatively high temperature (>400°C) and the
selectivity for 2-ethylhexanol was only 90.5% [19]. Thus, it
is necessary to develop an effective hydrogenation catalyst.
In this paper, the supported-metal catalysts with different
carriers were synthesized and used as catalysts in the hydro-
genation of 2-ethylhexenal to produce 2-ethylhexanol. To our
surprise, the catalyst Pd/ZrO2 had an excellent catalytic per-
formance and reusability in the hydrogenation reaction.
Hydrogen (99.9 wt%, Qingdao Tianfu Gas Co. Ltd.,
China), zirconyl chloride octahydrate (99.9 wt%, Shang-
hai Macklin Biochemical Co. Ltd., China), cerium (III)
nitrate hexahydrate (99.0 wt%, Shanghai Shanfu Chemical
Co. Ltd., China), aluminum oxide, MCM-41, SBA-15 and
MAS-7 (Suzhou Kangshuo Chemicals Co. Ltd., China),
cupric nitrate, zinc nitrate, ammonium dichromate, ruthe-
nium chloride, palladium chloride (99.9 wt%, Wuhan Feng-
fan Chemicals Co. Ltd., China), and other chemicals (ana-
lytical purity) were commercially available. All materials
were used without further purification.
2.2 Catalyst Preparation
The supported-metal catalysts were prepared by the deposi-
tion-precipitation method, and the detailed process for the
synthesis of Pd/ZrO2 was presented as follows: ZrO2 car-
rier was prepared by a coprecipitation method. For a typical
synthesis process, the dilute NH3·H2O solution (0.5 mol/L)
was added dropwise into 100 mL 0.2 mol/L ZrOCl2·8H2O
aqueous solution until the pH value of the mixed solution
reached 10 at 40°C obtaining the mother liquid. Then the
resultant precipitate was aged in the mother liquid for 12 h,
followed by filtration and washing with deionized water
and ethanol repeatedly to neutral. The obtained filter cake
was dried at 105°C for 6 h, and then calcined at 500°C for
4 h in air, giving ZrO2 powder. Pd/ZrO2was prepared by the
deposition-precipitation method. 2.0 g ZrO2 powder was
dispersed in 37.7 mL 0.005 mol/L PdCl2 aqueous solution.
0.5 mol/L Na2CO3 aqueous solution was added into until
the pH of the dispersion reached 10.5, and then the disper-
sion was aged at 60°C for 1 h under stirring which made
palladium hydroxide precipitate exclusively on the surface
of the ZrO2 carrier. The resulting solid was separated and
washed with distilled water several times to neutral, dried
for 6 h at 60°C and finally calcined at 300°C in air for 4 h,
obtaining the catalyst Pd/ZrO2 containing about 1.0 wt%
Pd [20, 21]. The CeO2 carrier was prepared in the similar
way, and the catalysts Pd/CeO2, Pd/MCM-41, Pd/MAS-7,
Pd/Al2O3, Pd/SBA-15 and Pd/C were prepared by the simi-
lar deposition-precipitation method. The catalysts Ni/ZrO2,
Co/ZrO2, Rh/ZrO2 and Ru/ZrO2 were also synthesized by
the deposition-precipitation method. It was noteworthy that
the catalysts Ni/ZrO2 and Co/ZrO2 were reduced in a 10%
H2 in N2 mixture at a heating rate of 5°C /min and dwelled
at 300°C for 4 h. The Cu/Zn catalyst was prepared by
the coprecipitation method. 100 mL 0.5 mol/L Zn(NO3)2
solution and 100 mL 0.5 mol/L Cu(NO3)2 solution were
mixed. 1.0 mol/L (NH4)2CO3 aqueous solution was added
1 3

Hydrogenation of 2-Ethylhexenal Using Supported-Metal Catalysts for Production of…
989
dropwise into the obtained mixture under vigorous stirring
until the pH of the reaction mixture was 10. The resulting
solution was aged at 30°C for 3 h. The obtained precipitate
was separated, washed 3 times with the distilled water, and
+ H
2
O
OH
Scheme 1 Hydrogenation of 2-ethylhexenal
dried at 105°C, then calcined at 400°C in air, achieving the
Cu/Zn catalyst. The Cu/Cr was prepared in the similar way.
The powder X-ray diffraction (XRD) patterns were
measured in the range from 5° to 75° using a Rigaku
D/max-RA powder diffractometer (Rigaku, Tokyo,
Japan) with Cu K radiation (λ=0.15418 nm). The
Brunauer–Emmer–Teller (BET) surface areas of the sam-
ples were determined using the nitrogen adsorption method
on a Micromeritics ASAP 2020 (Micromeritics Instrument
Co., Norcross, GA) at −196°C. The sample was degassed
under vacuum at 200°C for 4 h prior to adsorption analy-
sis. The high resolution transmission electron microscopy
(HR–TEM) measurements were performed using a high
resolution TEM JEOL 2100F at 200 keV. Temperature-
programmed reduction (H2-TPR) experiments were carried
out in a conventional system equipped with a thermal con-
ductivity detector (TCD). All the samples (100 mg) were
pretreated in a quartz U-tube in a flow of pure N2 at 400°C
for 45 min, then cooled. The reduction reaction was carried
out in a flow of 5% H2 in N2 from 50 to 900°C with a lin-
ear heating rate of 8°C/min. The metal contents in the sam-
ples were analyzed using a PerkinElmer Optima 5300 DV
(PerkinElmer, USA) inductively coupled plasma atomic
emission spectrometer (ICP-AES) system with a radio fre-
quency power of 1300 W.
Table 1 Effect of catalysts on the hydrogenation
Entry
Catalysts
C/%
SOL/%
SAL/%
1
2
3
4
5
6
7
8
Cu/Cr(1:1)
Cu/Zn(1:1)
PdCl2(0.02 g)
Pd/C
90.4
88.9
80.3
82.5
100
93.9
93.1
100
96.5
95.5
/
92.6
90.1
88.6
84.8
99.1
89.2
95.8
95.6
95.1
91.3
/
7.4
9.9
11.4
15.2
0.9
10.8
4.2
4.4
4.9
8.7
/
Pd/ZrO2
Pd/Al2O3
Pd/CeO2
Pd/MAS-7
Pd/MCM-41
Pd/SBA-15
ZrO2
Rh/ZrO2
Ni/ZrO2
Co/ZrO2
Ru/ZrO2
9
10
11
12
13
14
15
97.6
95.8
96.0
97.5
94.7
89.8
90.7
94.9
5.3
10.2
9.3
5.1
2-Ethylhexenal 2.0 g, catalysts 0.10 g, T=240°C, t=7 h, hydrogen
pressure (P) 6 MPa. SOL The selectivity of 2-ethylhexanol, SAL The
selectivity of 2-ethylhexanal
selectivity was calculated by: SOL%= WOL/WALL × 100,
and the 2-ethylhexanal (SAL%) selectivity was calculated
by: SAL%= WAL/WALL × 100, where WOL and WAL are the
amount of 2-ethylhexanol and 2-ethylhexanal, and WALL is
the total amount of the products, including 2-ethylhexanol,
2-ethylhexanal and isooctane, the selectivity of isooctane is
lower than 0.1% in all experiments. All experiments were
repeated four times in order to determine the reproduc-
ibility of the results. The reaction formula is showed in
Scheme 1.
2.3 Hydrogenation of 2–ethylhexenal
2.0 g 2-Ethylhexenal and 0.1 g Pd/ZrO2 were reacted in a
75 mL stainless-steel autoclave with 6 MPa H2 at 240°C for
7 h with stirring agitation (500 r/min), and then the reacted
mixture was cooled to room temperature and depressur-
ized. The gas phase material was collected in the air bag,
and the upper product was separated by decantation from
the catalyst layer. The catalyst layer was reused directly in
the recycle experiments. The gas phase material and the
liquid phase products were characterized qualitatively with
HP6890/5973 GC/MS equipped with an HP5–MS column,
30 m×0.25 mm×0.25 μm. No small molecule crack-
ing products were found in the gas phase. The products of
2-ethylhexanol, 2-ethylhexanal and isooctane were detected
in the liquid phase. The quantitative analysis of prod-
ucts were determined by GC using HP6890 GC equipped
with an HP5–MS column, 30 m×0.25 mm×0.25 μm,
the contents of the reactants and products were showed
by the system of GC chemstation according to the area
of each chromatograph peak. The 2-ethylhexenal conver-
sion was defined as C%, which is the wt% of 2-ethylhex-
enal consumed in the reaction. The 2-ethylhexanol (SOL%)
3 Results and Discussion
3.1 Effects of the Different Catalysts
on the Hydrogenation
As can be seen from Table 1, compared with traditional cat-
alysts (Entry 1, 2), the supported-metal catalysts exhibited
better catalytic performances. When Cu/Zn or Cu/Cr was
used as the catalyst, the utilization rate of 2-ethylhexenal
was generally low and 2-ethylhexanal was easily generated,
which means that the hydrogenation was not completely
carried out. Otherwise, the hydrogenation results were poor
1 3

990
G. Liu et al.
when PdCl2 or Pd/C was used as catalyst (Entry 3 and 4). It
is suggested that the above catalysts have the poor catalytic
performance for the hydrogenation of 2-ethylhexenal. The
different carriers had a significant influence on the catalytic
performance of Pd (Entry 5–10). Pd/ZrO2 showed the best
catalytic performance for the hydrogenation with 100%
conversion of 2-ethylhexenal and more than 99% selectiv-
ity for the desired product 2-ethylhexanol (Entry 5). The
results of the catalytic systems, such as Pd/Al2O3 and Pd/
CeO2, were not satisfied, though the carriers of the catalytic
active component Pd were metal oxides (Entries 6 and 7).
Otherwise, the catalytic performance of Pd catalyst system
supported by molecular sieve was also poor (Entries 8–10).
This may be because of superior dispersion of Pd on the
support ZrO2, the redox surface property of ZrO2 and the
synergistic effect of the bifunctional metal-support interac-
tion [22, 23]. The redox surface property of ZrO2 improves
the transfer of electrons, which also promotes the hydro-
genation reaction and the formation of the product [22]. On
the other hand, the surface atoms of the monoclinic ZrO2
have the good structure matching adaptability, and can well
match with the catalytic active species such as Pd, PdO
and PdO2. So the carrier ZrO2 controls the aggregation
morphology of the catalytic active species on the surface,
which promotes the dispersion of Pd component on the sur-
face of the unit carrier. In order further illustrate the best
dispersion of Pd on the carrier ZrO2, the Pd/ZrO2 (a), Pd/
SBA-15 (b), Pd/Al2O3 (c), Pd/MCM-41 (d), Pd/MAS-7
(e) and Pd/CeO2 (f) were characterized by TEM, and the
average particle sizes of Pd on the different supports were
quantified. As can been seen from Fig. 1, the average par-
ticle sizes of Pd on the support ZrO2 is 2.89 nm, while the
others on the ZrO2 are more than 3.00 nm, which indicates
that the ZrO2 support has an excellent dispersion perfor-
mance for Pd nanoparticles. So the catalytic performance
of Pd/ZrO2 was excellent.
When the hydrogenation of the 2-ethylhexenal reaction
was performed over the support ZrO2, it did not exhibit any
activity (Entry 11). Therefore, the result also certified the
synergistic catalytic performance of Pd and ZrO2. In order
to further illustrate the synergistic effect of the bifunctional
metal-support interaction, the samples of ZrO2 (a), unused
Pd/ZrO2 (b), and repeatedly used eight times Pd/ZrO2 (c)
were analyzed by the XRD, and the results are showed in
Fig. 2. As can be seen from Fig. 2a, b, the diffraction peaks
with 2θ at 30.3°, 35.2°, 50.2° and 60.2° were observed,
which indicates that the ZrO2 and the unused Pd/ZrO2
show mainly a tetragonal phase. The diffraction peaks of
3.12nm
3.18nm
2.89nm
a
b
c
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Diamter (nm)
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Diamter (nm)
Diameter(nm)
3.64nm
3.23nm
3.01nm
d
e
f
1.0 1.5 2.0 2.5 3.0 3.5 4.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Diamter (nm)
1.5 2.0 2.5 3.0 3.5 4.0 4.5
Diamter (nm)
Diameter (nm)
Fig. 1 The TEM images of Pd/ZrO2 (a), Pd/SBA-15 (b), Pd/Al2O3 (c), Pd/MCM-41 (d), Pd/MAS-7 (e), Pd/CeO2 (f)
1 3

Hydrogenation of 2-Ethylhexenal Using Supported-Metal Catalysts for Production of…
991
property, and the conversion of 2-ethylhexenal and the
selectivity for 2-ethylhexanol were less than 98% and 95%,
respectively. It is indicated that Pd/ZrO2 is an efficient cata-
lytic system for the carbon carbon double bond and alde-
hyde group [25]. The catalysts of Ni, Co, Ru, Rh supported
on ZrO2 (Entries 12–15) showed relatively high activity.
This may be due to that Ni, Co, Ru and Rh are commonly
catalytic centers for the hydrogenation with the good cata-
lytic performance. And ZrO2 can effectively promote the
dispersion of metals which makes the catalyst have high
activity.
t-ZrO
m-ZrO
Pd(PdOx)
∗
2
∗
∗
♦
2
♥
∗
∗
∗
∗
a
∗
∗
∗
b
♦
♦
♥
To further illustrate the highest activity of Pd/ZrO2, TPR
of the Pd/ZrO2 (a), Ni/ZrO2 (b), Co/ZrO2 (c), Rh/ZrO2 (d)
and Ru/ZrO2 (e) were characterized and the results are
showed in Fig. 3. As can be seen from Fig. 3, the shape of
the TPR curve, the size of the peak and the peak top tem-
perature were related to the composition of the catalyst and
the nature of the reducible species. In general, with increas-
ing the number of active sites in the catalyst, the initial peak
temperature of the reduction peak is lower and the peak
area is larger. Compared with the TPR curves of Pd/ZrO2
(a), Ni/ZrO2 (b), Co/ZrO2 (c), Rh/ZrO2 (d) and Ru/ZrO2
(e), the peak area of PdOx/ZrO2 TPR curve was larger even
its reduction temperature was low (240°C). Otherwise,
the contents of the active components were also measured
by ICP-AES, and the initial content was almost same for
each sample (Pd 0.98 wt%, Ni 0.97 wt %, Co 0.97 wt %, Rh
0.96 wt %, Ru 0.96 wt %).
∗
60
♦
♥
♥
♥
c
10
20
30
40
50
70
80
2θ(degrees)
Fig. 2 X-ray diffraction patterns of ZrO2 (a), unused Pd/ZrO2 (b) and
repeatedly used eight times Pd/ZrO2 (c)
the unused Pd/ZrO2were sharper than those of ZrO2, indi-
cating that palladium oxide is stabilized by the ZrO2 car-
rier. No distinct diffraction peak of Pd (2θ=40.5°), PdO
(2θ=34.0°) and PdO2 (2θ=54.6°) were observed from
Fig. 2b. The results may be due to that the Pd species are
highly dispersed on the surface of carrier ZrO2. While the
diffraction peaks of Pd were identified from Fig. 2c, it may
be due to the agglomeration of Pd particles during the reac-
tion which makes the metal expose on the surface of ZrO2.
The diffraction peaks with 2θ at 24.4°, 28.1°, 31.4°. 55.5°
assigned to monoclinic ZrO2 phase, which indicates that
the aged Pd/ZrO2 shows mainly a monoclinic phase [24].
Among the catalytic systems using ZrO2 as the carrier
(Entries 5, 12–15), Pd/ZrO2 exhibited the best catalytic
3.2 Effects of the Reaction Conditions
on the Hydrogenation
Figure 4a–d shows the effects of the catalyst dosage, reac-
tion temperature, reaction time and reaction pressure on
the hydrogenation using Pd/ZrO2 as catalyst. It was found
that the conversion of 2-ethylhexenal and the selectivity
for 2-ethylhexanol increased unpon increasing the cata-
lyst from 0.05 to 0.10 g. So an appropriate increase in the
amount of catalyst can effectively improve the hydrogen-
ation. When the catalyst dosage was 0.10 g, the conver-
sion of 2-ethylhexenal and the selectivity for 2-ethylhex-
anol were 100% and 98.0%, respectively. After that, no
significant increase in the 2-ethylhexanol selectivity upon
increasing the catalyst dosage could be obtained (Fig. 4a).
The reaction temperature had a significant effect on the
hydrogenation (Fig. 4b). With decreasing the reaction
temperature from 250 to 210 °C, the conversion of 2-eth-
ylhexenal expectedly decreased, while the selectively of
2-ethylhexanol first increased and then decreased, and it
was found that some alkane by-product was formed when
the reaction temperature was more than 250 °C. It was
suggested that too high a temperature enhanced the dehy-
droxylation of the hydroxyl compound. Therefore, the
a
b
c
d
e
100
200
300
400
500
)
600
700
Temperature (oC
Fig. 3 TPR profiles of Pd/ZrO2 (a), Ni/ZrO2 (b), Co/ZrO2 (c), Rh/
ZrO2 (d) and Ru/ZrO2 (e)
1 3

992
G. Liu et al.
a
b
100
80
60
40
20
0
100
80
60
40
20
0
C
C
SOL
SOL
SAL
SAL
210
220
230
240
250
0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22
Catalyst Dosage (g)
Temperature(oC
)
c
d
100
100
80
60
40
20
0
C
C
SOL
SOL
80
60
40
20
0
SAL
SAL
3
4
5
6
7
8
9
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Reaction Time (h)
Reaction Pressure (P)
Fig. 4 Effects of catalyst dosage (a), reaction temperature (b), reaction time (c) and reaction pressure (d) on the hydrogenation
optimal reaction temperature was 240 °C. The effect of
reaction time is also showed in Fig. 4c. With prolonging
the reaction time, the conversion of 2-ethylhexenal and
the selectivity for 2-ethylhexanol significantly increased.
When the reaction time was 7 h, the conversion of 2-eth-
ylhexenal and the selectivity for 2-ethylhexanol were 100
and 98.5%, respectively. However, both did not further
change when the time was increased further, and some
alkane by-product originating from the dehydroxylation
was found when the reaction time was more than 7 h. So
the optimal reaction time was 7 h. The H2 pressure also
has an important effect on the hydrogenation (Fig. 4d).
With increasing H2 pressure, the hydrogenation reaction
was easy to carry out, and the conversion of 2-ethylhex-
enal and the selectivity for 2-ethylhexanol were 100 and
99.1% respectively when the reaction was carried out at
240 °C under H2 pressure 6 MPa. Otherwise the alkane
byproduct was not detected. Based on the above results,
the optimum reaction conditions were obtained as fol-
lows: 2-Ethylhexenal 2.0 g, Pd/ZrO2 0.10 g, H2 pressure
6 MPa, reaction temperature 240 °C and reaction time7 h.
Under these conditions, the conversion of 2-ethylhexenal
Table 2 Reusability of the catalyst
Times
C/%
SOL/%
SAL/%
1
2
3
4
5
6
7
8
100
100
100
99.8
99.6
99.7
92.6
86.9
98.9
98.2
98.0
98.2
97.5
97.0
93.7
90.1
1.1
1.8
2.0
1.8
2.5
3.0
6.3
9.9
2–Ethylhexenal 2.0 g, Pd/ZrO2 0.10 g, T=240°C, t=7 h, P=6 MPa.
1 3

Hydrogenation of 2-Ethylhexenal Using Supported-Metal Catalysts for Production of…
993
a
b
c
2.89nm
3.38nm
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
1
2
3
4
5
6
Diameter(nm)
Diameter(nm)
a
b
c
Fig. 5 N2 adsorption–desorption isotherms of ZrO2 (a), fresh Pd/ZrO2 (b) and aged Pd/ZrO2 (c)
and the selectivity of 2-ethylhexanol were 100 and 99.1%,
respectively.
ZrO2, BET and TEM analysis were used to test the samples
of ZrO2 (a), unused Pd/ZrO2 (b), and repeatedly used eight
times Pd/ZrO2 (c), and the results are showed in Fig. 5 and
Table 3 and Fig. 6.
3.3 Reusability of Catalyst
The result of the N2 adsorption-desorption isotherms is
showed in Fig. 5, and the results of the BET surface area,
pore volume and pore diameter of catalysts are showed
in Table 3. The pore volume and pore size are calculated
based on the BJH method. As shown in Fig. 5, the adsorp-
tion-desorption isotherms of three samples appear capillary
condensation system, which were the typical mesoporous
material isotherms, indicating that the mesoporous struc-
ture of the repeatedly used eight times Pd/ZrO2 is not
destroyed. As shown in Table 3, the specific surface area
of ZrO2 was 68.18 m2/g, but that of the unused Pd/ZrO2
decreased slightly to 65.55 m2/g. It may be attributed to
that the Pd oxides support on the carrier ZrO2. When Pd/
ZrO2 was repeatedly used eight times, the specific surface
area, the pore volume and diameter decreased significantly,
which implies that the coking of catalyst blocks the partial
pores [26].
When the reaction was finished, the upper organic phase
was separated by centrifugation. Then by adding fresh rea-
gent, Pd/ZrO2 was directly used to investigate its reusability
under the optimum reaction conditions. Results are showed
in Table 2. The catalytic system was reused six times with-
out any obvious decrease in the conversion and selectiv-
ity, which indicates that the catalyst system is reusable for
multiple times. However, the conversion and selectivity
slightly decreased for eight times, which may be because of
the aggregation of Pd particles and the coking of ZrO2 dur-
ing the reaction process. So the preparation and anti-coking
capability of carrier is still a lot of work to do. In order to
clarify the aggregation of Pd particles and the coking of
Table 3 The characterization results of (ZrO2 (a), Pd/ZrO2 (b) and
aged Pd/ZrO2 (c))
The TEM images are showed in Fig. 6. As can be seen
from Fig. 6a, a′, b, b′, the particle sizes of ZrO2 and the
unused Pd/ZrO2 changed slightly, and they were all in
nanometer level which indicated that the carrier was sta-
ble in the process of catalyst preparation. The images of
Fig. 6b, b′ showed that ZrO2 was spherical or ellipsoidal,
and the Pd particles were supported on the ZrO2 carrier.
Samples
SBET (m2/g)
Pore volume
(cm3/g)
Pore
diameter
(nm)
a
b
c
68.18
65.55
46.20
0.24
0.20
0.14
3.00
2.96
1.58
1 3

994
G. Liu et al.
a
b
c
2.89nm
3.38nm
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
1
2
3
4
5
6
Diameter(nm)
Diameter(nm)
a
b
c
Fig. 6 TEM images of ZrO2 (a, a′), unused Pd/ZrO2 (b, b′) and repeatedly used eight times Pd/ZrO2 (c, c′)
The average particle size of the unused ZrO2 was 15–25 nm
and the average particle size of Pd was 2.89 nm. While the
obtained micrographs of the repeatedly used eight times
Pd/ZrO2 revealed that the samples bore agglomerated
microstructures, and the result was good agreement with
the result of XRD. In particular, the size of the Pd particles
was 3.38 nm when Pd/ZrO2 was repeated used eight times,
which was larger than the particle size of the unused Pd/
ZrO2. This phenomenon is mainly ascribed to the aggrega-
tion of the Pd particulates.
the coking of ZrO2 the catalysts were the main cause for the
deactivation of the catalyst.
Acknowledgements This work was financially supported by
the National Basic Research Program of China (973 Program)
(2014CB460610), the National Natural Science Foundation of China
(No. 30571463) and the TaiShan Scholars Projects of Shandong (No.
ts201511033). The authors are grateful for the financial support.
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