Effects of Ir content on selective hydrogenation of crotonaldehyde over Ir/ZrO2 catalysts

Article abstract of DOI:10.1016/j.catcom.2012.01.019

A series of Ir/ZrO₂ catalysts with different Ir contents were tested for gas phase selective hydrogenation of crotonaldehyde. It was found that the reactivity of the catalyst increased with increasing Ir content, but rapid deactivation was obse

Full text of DOI:10.1016/j.catcom.2012.01.019

Catalysis Communications 21 (2012) 5–8
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Effects of Ir content on selective hydrogenation of crotonaldehyde
over Ir/ZrO2 catalysts
⁎
Lin Zhu, Geng-Shen Hu, Ji-Qing Lu, Guan-Qun Xie, Ping Chen, Meng-Fei Luo
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Ir/ZrO2 catalysts with different Ir contents were tested for gas phase selective hydrogenation of
crotonaldehyde. It was found that the reactivity of the catalyst increased with increasing Ir content, but
rapid deactivation was observed on all the catalysts during the reaction, which might be due to CO poisoning
generated by decarbonylation reaction. The highest selectivity (82.2%) to crotyl alcohol was obtained over a
catalyst with 3% Ir content, which was attributed to the proper Ir particle size and the highest amount of sur-
face acid sites in the catalyst.
Received 14 November 2011
Received in revised form 11 January 2012
Accepted 16 January 2012
Available online 23 January 2012
Keywords:
Ir/ZrO2 catalyst
© 2012 Elsevier B.V. All rights reserved.
Crotonaldehyde hydrogenation
Crotyl alcohol
Lewis acid sites
1. Introduction
also found that the presence of partially reducible supports such as
ZrO2 and TiO2 could enhance the selectivity to crotyl alcohol [12].
Crotonaldehyde (CH3CH_CHCHO) is a typical α, β-unsaturated
aldehyde, and its main hydrogenation products are crotyl alcohol
(hydrogenation of C_O bond), butanal (hydrogenation of C_C
bond) and butanol (hydrogenation C_C and C_O bonds). Crotyl al-
cohol (CH3CH_CHCH2OH), the product of C_O hydrogenation is
highly desired because it has wide applications in the synthesis of
fine chemicals, such as pharmaceutical, agrochemical, and fragrance
compounds [1–3]. However, chemoselective hydrogenation of α,
β-unsaturated aldehydes to unsaturated alcohols is more difficult,
because the hydrogenation prefers to C_C bond compared with
C_O bond for both thermodynamic and kinetic factors [4].
It appears that the Ir systems are promising for selective hydroge-
nation of crotonaldehyde, therefore, these Ir catalysts deserve a
systematic investigation because it is well known that catalytic reac-
tivity of the catalyst for this reaction depends on various parameters
such as preparation method, metal precursor, support type, particle
size of active metal, and pretreatment conditions [13]. However, the
correlationship between the Ir catalysts and catalytic behaviors has
been rarely reported. More importantly, catalyst deactivation is com-
monly observed for this reaction [14,15], and thus the mechanism of
deactivation is also worthy for investigations in order to design cata-
lyst systems with better performance.
Noble metal catalysts show good performance for crotonaldehyde
hydrogenation. Pt catalysts have been intensively studied, such as Pt/
Ga2O3 [5], Pt/TiO2 [6], Pt/Ta2O5/ZrO2 [7], Pt/ZnO [8], and Pt/SnO2 [9].
Although most attention has been devoted to Pt catalysts, recently,
another noble metal Ir catalysts are also found to be quite high selec-
tivity to crotyl alcohol for gas-phase hydrogenation of crotonalde-
hyde. Abid et al. [10] reported the catalytic performance of 5%Ir/
TiO2 catalysts after high temperature calcination for gas phase hydro-
genation of crotonaldehyde with selectivity to crotyl alcohol of 70%
at 20–80% conversion. Reyes et al. [11] found that Ir/TiO2 catalysts
reduced at high temperature showed high activity due to a strong
metal support interaction (SMSI) effect. Moreover, the same authors
Therefore, in this work, a series of Ir/ZrO2 catalysts with different
Ir contents were prepared and tested for selective hydrogenation of
crotonaldehyde. The relationship between the catalyst features and
catalytic behaviors was established and the deactivation of the cata-
lysts was discussed.
2. Experimental
2.1. Catalyst preparation
ZrO2 was prepared from ZrOCl2 by a precipitation method. An NH3
aqueous solution was added to a ZrOCl2 solution under constant
stirring. The final pH value of the resulting mixture was controlled
at 10 0.5. The mixture was kept stirring at room temperature for
24 h and aged at 90 °C for 5 days. Then it was filtered and washed
with deionized water. The achieved white powder was dried at
60 °C for 8 h under vacuum, followed by calcination at 400 °C for
⁎
Corresponding author. Tel.: +86 579 82283910; fax: +86 579 82282595.
E-mail address: mengfeiluo@zjnu.cn (M.-F. Luo).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.01.019

6
L. Zhu et al. / Catalysis Communications 21 (2012) 5–8
4 h, and the final ZrO2 support with a specific surface area of
313 m2 g−1 was obtained.
The Ir/ZrO2 catalyst was prepared by impregnating the ZrO2
support with an aqueous solution of H2IrCl6. After impregnation for
12 h, the mixture was dried overnight at 90 °C for 8 h. The catalysts
were denoted as xIr/ZrO2, where x refers to the Ir content in the
catalyst (wt.%).
2.2. Catalyst characterizations
All the catalysts were reduced in H2 at 500 °C for 1 h prior to
characterizations.
X-ray diffraction (XRD) patterns were recorded using a PANalytic
X'Pert PW3040 diffractiometer. Elemental compositions were deter-
mined by X-ray fluorescence (XRF) analysis, using an ARLADVANT'X
Intelli Power 4200 scanning X-ray fluorescence spectrometer. Surface
areas of the catalysts were determined by the modified BET method
from the N2 sorption isotherms at −196 °C on an Autosorb-1 appara-
tus. All the samples were degassed at 100 °C prior to measurements.
CO chemisorption experiments were carried out on a Quantachrame
CHEMBET-3000 instrument in order to determine the dispersion of
Ir and the particle size. Ammonia temperature-programmed desorp-
tion (NH3-TPD) was conducted on a home-made apparatus, and the
total amount of NH3 desorbed was determined by reaction with an
excess of dilute hydrochloric acid and back titration with sodium
hydroxide solution.
Fig. 1. XRD patterns of Ir/ZrO2 catalysts.
that the 3Ir/ZrO2 catalyst has the strongest surface acidity. Also,
the amount of surface acid sites increases with Ir content, and reaches
a maximum (3.68 mmol/g) when the Ir content is 3% (3Ir/ZrO2).
Further increase of Ir content results in a decline in surface acid
sites. This is because the surface acid sites might be covered by excess
Cl species. Therefore, the 3Ir/ZrO2 catalyst has the strongest surface
acidity and the largest amount of surface acid sites.
2.3. Catalytic testing
The gas phase hydrogenation of crotonaldehyde was performed
in a fixed bed reaction system at atmospheric pressure, using a
quartz tubular (8 mm i.d.) reactor. Before running the test, the cata-
3.3. Ir/ZrO2 catalysts for selective hydrogenation of crotonaldehyde
Fig. 3 shows the selective hydrogenation of crotonaldehyde over
the 3Ir/ZrO2 catalyst reduced at 500 °C. It is found that the catalyst
is quite active at the initial stage, with a crotonaldehyde conversion
of 46%. However, the catalyst gradually deactivates with time on
stream and reaches a quasi-steady state after 400 min. Such deactiva-
tion is commonly observed for the hydrogenation of crotonaldehyde
[17,18]. Concerning the selectivity to crotyl alcohol, it shows a rapid
growth during the first 100 min of reaction, and then reaches a stable
level (82.2%) after 350 min.
lyst was reduced at 500 °C for 1 h in ultra-pure H2 (20 ml min−1
)
and then it was cooled down to 60 °C. Crotonaldehyde was intro-
duced to the reactor by a H2 flow (26 ml min−1) passing through a
crotonaldehyde saturator maintained at 0 °C to achieve a constant
crotonaldehyde partial pressure (1.06 kPa). The reaction products
were analyzed using an on-line gas chromatography (Shimadzu GC-
2014) equipped with a flame ionization detector (FID) and a DB-
Wax column (30 m×0.25 mm×0.25 μm) capillary column.
Also seen in Fig. 3, small amount of C3 hydrocarbons are produced
during the reaction, indicating the occurrence of decarbonylation
reaction and the simultaneous formation of CO [19]. It was reported
that CO molecules could be strongly adsorbed on the Ir surface and
the active sites are blocked, which leads to a partial loss of active
sites [20]. In order to further confirm whether the deactivation is
caused by CO poisoning, 1 ml CO was injected into reactor before
the reaction. Fig. 4 shows the effect of CO poisoning on the catalytic
behavior of the 3Ir/ZrO2 catalyst. It can be seen that the crotonalde-
hyde conversion has a dramatic decline (from 51 to 19%) at the initial
stage after CO adsorption, and the reactivity is obviously suppressed
by the poisoning during the whole reaction. Therefore, it can be con-
cluded that the strong CO adsorption on the surface of Ir atoms is
3. Results and discussion
3.1. Composition and texture of Ir/ZrO2 catalysts
Fig. 1 shows the XRD patterns of the Ir/ZrO2 catalysts. It can be
seen that the diffraction peaks of the catalysts are similar to that of
the ZrO2 support, and no peaks related to Ir species are observed,
implying that the Ir species are highly dispersed. Table 1 lists the Ir,
Cl contents, and specific surface areas of the catalysts. It is found
that the Ir contents in all catalysts are close to the nominal Ir values.
Also, Cl species are still present in the catalysts even after reduction
at 500 °C, and the Cl content increases with increasing Ir content. In
addition, the surface area of the catalyst decreases with Ir content in
the catalyst which could be attributed to the factor that the pores of
ZrO2 may be partially blocked by Ir species [16]. In addition, the dis-
persion of Ir particles decreases with increasing Ir content, and the
average particle size of Ir grows from 1.5 to 6.8 nm.
Table 1
Physical properties of Ir/ZrO2 catalysts.
Catalyst
BET specific
surface area
(m2/g)
Dispersion
of Ira (%)
Particle size
of Ira (nm)
Content(wt.%)
Ir
Cl
3.2. Surface acidity of Ir/ZrO2 catalysts
ZrO2
313
289
282
274
262
253
–
–
–
–
1Ir/ZrO2
2Ir/ZrO2
3Ir/ZrO2
4Ir/ZrO2
5Ir/ZrO2
65.8
33.3
25.2
21.1
14.6
1.5
2.9
3.9
4.7
6.8
1.23
2.29
3.26
4.43
5.54
1.21
2.04
2.55
2.81
2.92
Fig. 2 shows the NH3-TPD profiles of the Ir/ZrO2 catalysts. It can be
seen that the Ir/ZrO2 catalysts and the ZrO2 support have two NH3
desorption peaks at around 210 (α) and 415 °C (β). With increasing
Ir content, both the α and β peaks shift to higher temperature and
reach maxima (456 °C) when the Ir content is 3%. This indicates
a
Measured by CO chemisorption, assuming that CO: Ir=1.

L. Zhu et al. / Catalysis Communications 21 (2012) 5–8
7
Fig. 4. Effect of CO poisoning on the crotonaldehyde hydrogenation over 3Ir/ZrO2
Fig. 2. NH3-TPD profiles of Ir/ZrO2 catalysts.
catalyst (o, -fresh catalyst; ⋄♦-catalyst pretreated with CO).
•
responsible for the dramatic and irreversible deactivation of the Ir/
ZrO2 catalyst. Similar deactivation was also observed over Pt catalysts
and attributed to the strong CO chemisorption over surface Pt atoms
[13,21].
Table 2 lists the catalytic activities of the Ir/ZrO2 catalysts obtained
at 60 and 300 min of reaction. All the catalysts suffer deactivation
during the reaction. With increasing Ir content, the quasi-steady
state conversion of crotonaldehyde and TOF increases gradually.
When the Ir content is 5%, the conversion of crotonaldehyde is
31.6% after 300 min of reaction.
Concerning the selectivity, it is noticed in Table 2 that the selectiv-
ity to crotyl alcohol is much higher (75–82%) on the catalysts with
high Ir contents (> 1%) than that with a Ir content of 1% (58%),
which reflects the influence of Ir particle size on the selectivity. Eng-
lisch et al. [20] suggested that extended low index planes (e.g. {111})
on large Pt particles provided the sites that favored crotyl alcohol
formation. In the case of Ir/ZrO2 catalysts, similar effect of Ir particle
size may also exist since the Ir particles grow up with increasing con-
tents (Table 1), although detailed studies of the exposing planes of Ir
particles in these catalysts (such as HRTEM) are very necessary to ob-
tain solid evidence. However, it is found that the selectivity slightly
decreases with further increasing Ir content (e.g. 4 and 5%), implying
factors that influence the selectivity other than Ir particle size. It has
been reported that the surface Lewis acid sites in the catalyst are ben-
eficial to the adsorption of crotonaldehyde via carbonyl group, and
thus improve the selectivity of crotyl alcohol [22,23]. In the present
work, it is found that the surface acidity of ZrO2 was even enhanced
by the addition of Ir because of the acidic precursor H2IrCl6 (Fig. 2).
Therefore, based on the results reported in literature [24,25] and
our current work, an adsorption model of crotonaldehyde on Ir/ZrO2
catalyst is proposed, as shown in Scheme 1. The carbonyl oxygen is
suggested to strongly interact with Lewis acid sites (σ2) and the Ir
particle is the adsorption center of C_C band (π) and carbonyl carbon
(σ1). According to the NH3-TPD results (Fig. 2), for the Ir/ZrO2 cata-
lyst, the numbers and strength of surface acid sites are relevant to Ir
content, and the highest amount of acid sites is observed on the 3Ir/
ZrO2 catalyst. In this case, the terminal oxygen of crotonaldehyde
could strongly interact with the Lewis acid sites and this interaction
leads to high selectivity to crotyl alcohol.
When the Ir content is higher than 3% (e.g. 5%), although the σ1
bond is less intense due to the weak acidity, the Ir particles grow
from 3.9 to 6.8 nm, so that C_C bond and the carbonyl carbon
could be easily chemisorbed on Ir0 sites, and thus leads to a high re-
activity. Additionally, according to the crotonaldehyde adsorption
model (Scheme 1), the formation of crotyl alcohol takes place on
the interfacial region and involves both Ir and Lewis acid sites. The
growth of Ir particle results in a decline in the amount of interfacial
sites of Ir contacting with Lewis acid sites, while the sites for C_C
Fig. 3. Crotonaldehyde conversion and the selectivities to hydrogenation products as a
function of time on stream over 3Ir/ZrO2 catalyst.

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L. Zhu et al. / Catalysis Communications 21 (2012) 5–8
Table 2
Effect of the Ir contents on the performance of Ir/ZrO2 catalystsa.
Catalyst
Conversion
(%)
TOFb
Selectivity (%)
(×10−3 s−1
)
Crotyl alcohol
Butanal
Butanol
Othersc
1Ir/ZrO2
2Ir/ZrO2
3Ir/ZrO2
4Ir/ZrO2
5Ir/ZrO2
6.0(12.7)
12.2(34.9)
16.0(40.8)
25.2(59.1)
31.6(59.4)
2.9(6.1)
6.2(17.8)
7.6(19.3)
10.5(24.6)
15.2(70.6)
58.2(55.7)
77.0(66.5)
82.2(70.5)
76.2(56.6)
75.5(54.1)
21.6(17.6)
13.2(8.5)
8.1(5.4)
10.1(6.9)
9.8(7.0)
4.6(10.0)
6.1(19.5)
7.1(20.0)
9.5(31.8)
10.8(33.4)
15.6(16.7)
3.7(5.5)
2.7(4.1)
4.2(4.6)
3.9(5.5)
a
Experimental data were taken at 300 min or (60 min).
Calculated based on Ir dispersion by CO chemisorption results.
Others include C3 and C8.
b
c
Scheme 1. Proposed adsorption model of crotonaldehyde on Ir/ZrO2 catalyst.
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would result in a lower selectivity to crotyl alcohol. Therefore, the
suppressed selectivity to crotyl alcohol could be attributed to the pro-
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4. Conclusion
In summary, Ir/ZrO2 catalysts showed high activities for crotonal-
dehyde hydrogenation and high selectivities to crotyl alcohol. With
increasing Ir content in the catalyst, the crotonaldehyde conversion
gradually increased. However, all the catalysts suffered deactivation
during the reaction, which may be due to the CO poisoning caused
by decarbonylation reaction. Also, it is found that both the Ir particle
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Acknowledgments
This work was supported by the National Science Foundation of
China (Grant No. 21173194) and the Natural Science Foundation of
Zhejiang Provincial of China (Grant No. Y4100300).
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