Chemoselective hydrogenation of α,β-unsaturated aldehydes with modified Pd/C catalyst

Article abstract of DOI:10.1016/j.cclet.2012.05.002

Selective hydrogenation of α,β-unsaturated aldehydes with modified Pd/C catalyst was developed. The reduction of CO bond could be efficiently inhibited by the addition of carbonates, and high selectivity to the corresponding saturated aldehydes was achieved under mild conditions. This protocol provides an alternative for efficient preparation of saturated aldehydes.

Full text of DOI:10.1016/j.cclet.2012.05.002

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Chinese Chemical Letters 23 (2012) 773–776
www.elsevier.com/locate/cclet
Chemoselective hydrogenation of a,b-unsaturated aldehydes
with modified Pd/C catalyst
*
Wen Qiang Du, Ze Ming Rong , Yan Liang, Yong Wang, Xin Yi Lu,
Yi Fan Wang, Lian Hai Lu
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
Received 3 March 2012
Available online 9 June 2012
Abstract
Selective hydrogenation of a,b-unsaturated aldehydes with modified Pd/C catalyst was developed. The reduction of C O bond
could be efficiently inhibited by the addition of carbonates, and high selectivity to the corresponding saturated aldehydes was
achieved under mild conditions. This protocol provides an alternative for efficient preparation of saturated aldehydes.
# 2012 Ze Ming Rong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Pd/C catalyst; a,b-Unsaturated aldehydes; Hydrogenation; Selectivity
Saturated aldehydes have attracted great attention during the past decades [1] owing to their wide application in
perfume chemistry and synthetic chemistry [2]. 3-Phenylpropionaldehyde (PPA) and its derivatives could be prepared
by the selective hydrogenation of a,b-unsaturated aldehydes (UA). However, selective reduction on a C C double
bond is quite challenging due to the undesired hydrogenation of aldehyde group in the conjugate system. Compared
with other group-VIII transition metals, Pd was the relatively most effective catalyst for the hydrogenation of C C
double bond. Zhang et al. [3] employed the PdCl2–0.5Co(OAc)2–PPh3 reduced by NaBH4 as catalyst for the
hydrogenation of 3-phenylpropenal (PP) and the selectivity to PPAwas up to 97%. Tin et al. [4] studied the effect of pH
value of the solution on the hydrogenation of PP by water-soluble palladium complex PdCl2(TPPTS)2 and found that
the addition of Na2CO3 solution could promote the reduction of C C bond. Yet, it was still difficult to control the
selectivity of PPA. Water-soluble ruthenium complexes containing ligand TPPTS was also reported for selectively
reduction of PP and the selectivity to PPA reached 97% under optimal conditions [5]. Compared with homogeneous
catalysts, the heterogeneous catalysts are more environmentally friendly, not air/moisture sensitive, easier to be
separated and reused. Li et al. studied Pt–Ni/CNTs catalysts for the hydrogenation of PP, and the selectivity of PPA
increased to 96% when NaOAc was used as basic promoter [6]. In addition, Au [7], Pd [8], Ni [9], Cu [10] and
bimetallic catalysts [11] were also investigated for the PP hydrogenation to PPA, but either conversion efficiency or
reaction conditions remained unsatisfied. Herein, we report a simple one-pot method for highly selective
hydrogenation of UA to saturated aldehydes over Pd/C modified by carbonates.
* Corresponding author.
E-mail address: zeming@dlut.edu.cn (Z.M. Rong).
1001-8417/$ – see front matter # 2012 Ze Ming Rong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.05.002

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W.Q. Du et al. / Chinese Chemical Letters 23 (2012) 773–776
1. Experimental
The Pd/C (5 wt%) catalyst was prepared by incipient wetness impregnation with palladium acetate acetonic
solution. In brief, Pd(OAC)2 (0.223 mmol, Heraeus Materials Technology Shanghai Ltd., China) was added to 20 mL
acetone (AR) containing proper amount of proplyamine (AR) as promoter. The mixed solution was stirred at 323 K
until it was turned to a uniform and transparent system, and then 1 g activated carbon (dried at 453 K for 4 h, Shanghai
Activated Carbon Co., Ltd.) was added to the solution at room temperature with vigorous stirring for 1 h. 5 times
excess amount of NaBH4 (0.558 mmol, Tianjin Fuchen Chemical Reagents Factory) in 50 mL H2O was dropwise
added to the solution under stirring. The reduction time lasted for at least half an hour. After continually stirring for 1 h,
5 wt% Pd/C catalyst was obtained by filtration, washed with ethanol and hot deionized water for several times. Then
the samples of Pd/C catalysts were stirred with varying additives in methanolic solution for 1 h at ambient temperature.
The selective hydrogenation of a,b-unsaturated aldehydes was carried out in a stainless-steel autoclave (75 mL
volume) equipped with a magnetic stirrer. The reactor was quantitatively charged with catalyst (pretreated by
additive), substrate and solvent, after that the reactor was closed and flushed three times with N2 and H2 respectively.
When the autoclave was reached desired temperature, it was pressurized with hydrogen to set-point pressure that was
maintained a constant value during the reaction period. The reaction time was measured from starting the stirring. The
conversion and selectivity of product were analyzed by gas chromatograph (Agilent 6890N) equipped with a HP-5
capillary column (30 m  0.32 mm  0.5 mm) and flame ionization detector. The components of the reaction mixture
were identified by GC–MS (HP 6890GC/5973MSD).
2. Results and discussion
The hydrogenation of UA is a typical competitive reaction (Scheme 1) and the hydrogenation of the conjugated
C C bond and C O bond occurs through two parallel reactions. Saturated aldehyde (SA) and unsaturated alcohol
(UAO) are intermediates respectively, and saturated alcohol (SAO) is the complete hydrogenation product.
We first chose p-tert-butyl-a-methyl-3-phenylpropenal (TBMP) as a substrate to screen reaction parameters.
During our investigations toward TBMP hydrogenation, Pd/C catalyst was reactive at 313 K under 0.5 MPa of H2 but
the selectivity to p-tert-butyl-a-methyl-3-phenylpropionaldehyde (TBMPA) was very low (Table 1, entry 1). Adding
Na2CO3 additive directly to the reaction system proved to be beneficial for high conversion (Table 1, entry 2) and the
selectivity was up to 89.8%, which was much higher compared with the blank test. The selectivity to TBMPA was
further improved to 94.7% after Pd/C was pretreated with Na2CO3 1 h (Table 1, entry 3). Other carbonates as
promoters, such as NaHCO3, Li2CO3, K2CO3, Cs2CO3 (Table 1, entries 4–7) were also investigated, and K2CO3
provided the highest TOF of 103.4 hÀ1, which is twice as high as that in blank test. Further studies indicated that the
selectivity to TBMPA were not obviously improved when other additives such as (NH2)2CO3, (NH2)2CO,
NH2(CH2)2OH and NH3ÁH2O were used (Table 1, entries 8–11).
We speculated that competitive absorption of the C C and C O bond likely occurred in the catalyst’s surface. The
selectivity to TBMPA was significantly improved after adding the carbonates, which was easily adsorbed on the
catalyst surface due to interaction between the electron-rich of CO32À ions and the unoccupied 5s molecular orbital of
O
O
R2
R2
R1
R1
UA
SA
OH
OH
R2
R2
R1
R1
SAO
UAO
R1= H, C(CH3)3; R2= H, CH3, CH2CH2CH2CH2CH3
Scheme 1. Reaction pathway of UA hydrogenation.

W.Q. Du et al. / Chinese Chemical Letters 23 (2012) 773–776
775
Table 1
Selective hydrogenation of TBMP over Pd/C modified by additives.
Entry
Additive
t/min
Conv./%
Sele./%
SA
TOF/hÀ1
UAO
SAO
1
2
None
130
75
96.5
100
32.4
89.8
94.7
77.7
70.1
97.8
97.9
39.2
40.9
22.9
30.5
14.8
0.3
0.5
18.8
3.4
0.3
0.4
0
43.4
9.9
50.5
74.6
*
Na2CO3
Na2CO3
#
3
65
99.8
99.8
99.7
99.7
99.7
100
4.8
81.8
#
4
NaHCO3
#
Li2CO3
120
152
50
3.5
43.8
5
26.4
1.9
42.2
#
6
K2CO3
103.4
73.7
#
7
Cs2CO3
70
1.7
#
8
(NH2)2CO3
(NH2)2CO#
60
60.8
59.1
68.2
69.5
136.4
152.1
492.8
95.8
9
53
99.6
91.2
100
0
10
11
NH2(CH2)2OH#
15
0
NH3ÁH2O#
90
0
Reaction conditions: 313 K, 0.5 MPa, TBMP 5 mmol, CH3OH 20 mL, 5 wt% Pd/C 0.125 g, n(additive) 0.623 mmol. *: additive was directly added
to the reaction system; #: Pd/C was pretreated in the methanol with additive for 1 h, then the substrate was added to the reaction system. The
conversion and selectivity were determined by GC. TOF = TON/time, TON was molar ratio of converted functional group (C C bond and C
bond) and catalyst (Pd).
O
Pd. Thus, the adsorption of C O bond on the catalyst surface was suppressed due to the higher electron density of Pd/
C. This suggests that C C bond binds preferentially to the Pd center than C O, which results in the increased
selectivity of TBMPA. We then investigated the effect of different solvents such as methanol, ethanol, i-propanol,
tetrahydrofuran, cyclohexane (Table S1, see the supporting information) and methanol solvent provided the selectivity
up to 98.4%.
We also found that alkaline conditions might be favorable for the preparation of TBMPA. NaOH was used as an
additive for the hydrogenation of TBMP and results showed that Pd/C pretreated by NaOH was not as effective as that
by carbonates. However, the selectivity was increased when NaOH was directly added to the solution. The effect of the
amount of NaOH on the hydrogenation of TBMP was studied (Table S2, see the supporting information). The results
exhibited that the introduction of trace NaOH could not only enhance the reactivity, but also improve the selectivity for
the C C hydrogenation. When the NaOH amount was 7À9 mg scale, the maximum selectivity exceeded 95%.
Employing Pd/C catalyst pretreated by K2CO3 and the methanol solvent, effect of temperature, pressure and initial
concentration of TBMP were investigated (Table S3À5, Fig. S1À3, see the supporting information). Under optimal
reaction conditions, the concentration of components during the reaction process was monitored by GC and the
relationships between concentration and reaction time were shown in Fig. S4 (see the supporting information). It is
clear that the hydrogenation of TBMP occurs consecutively and competitively with TBMPA as a stable intermediate
and p-tert-butyl-b-methyl-3-phenylpropanol (TBMPP) as the final product. The concentration of TBMP decreases
almost linearly with reaction time, suggesting that the reaction has dynamically pseudo-zero order dependence with
TBMP. The hydrogenation rate of TBMP to TBMPA is much higher than that from TBMP to p-tert-butyl-b-methyl-3-
phenylprop-2-en-1-ol (TBMPO). Therefore it is plausible to stop the reaction at the intermediate step with TBMPA as
the main product. As the concentration of TBMP decreases, the amount of TBMPA increases almost linearly and the
selectivity is up to 97.8% with 99.7% TBMP conversion.
Having optimized the reaction conditions, we then investigated a series of substrates with similar conjugated
system (Table 2). Unlike the hydrogenation of TBMP, the reaction time was extended for the selective hydrogenation
of 3-phenylpropenal, a-methyl-3-phenylpropenal, a-pentyl-3-phenylpropenal (Table 2, entries 2–4, the reaction
mechanism refers to the supporting information). As can be seen from Table 2, using K2CO3 as additive to pretreat Pd/
C could effectively increase the selectivity of SA in the conjugate system compared with untouched catalyst,
indicating the reaction tolerates well for substrates with/without a tert-butyl group on para position of phenyl group or
an alkyl group on a position.
In summary, we have developed a highly selective and practical hydrogenation procedure for preparing saturated
aldehydes with Pd catalyst supported on activated carbon. With carbonates as effective additives, the selectivity of
C C hydrogenation of UA to saturated aldehydes was as high as 98.1%. Further investigation and application of the
catalyst system is still underway.

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W.Q. Du et al. / Chinese Chemical Letters 23 (2012) 773–776
Table 2
Selective hydrogenation of UA.
Entry
t/min
Conv./%
96.5
Sele./%
SA
UAO
14.8
SAO
43.4
130*
32.4
O
50#
6*
99.7
100
97.8
25.3
0.3
1.9
13.4
61.3
O
O
100#
12*
98.4
99.1
91.8
25.9
0
8.2
3.2
69.3
50#
25*
99.4
100
98.1
31.7
0
1.9
10.5
56.8
O
50#
99.8
97.6
0.2
1.9
Reaction conditions: 313 K, 0.5 MPa, substrate 5 mmol, CH3OH 20 mL, 5 wt% Pd/C 0.125 g. The conversion and selectivity were determined by
GC. *: Pd/C was used directly. #: Pd/C was pretreated in the methanol with 0.623 mmol K2CO3 for 1 h.
Acknowledgments
This work was financially supported by the National High Technology Research and Development Program of
China (863 Project) (No. 2007AA03Z345), and the Scientific Research Project for Institute of Higher Education of
Education Bureau, Liaoning (No. LT2010021) and the Fundamental Research Funds for Dalian University of
Technology (No. DUT10RC (3)107).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/
j.cclet.2012.05.002.
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