Durability enhancement of chirally modified metallic nickel catalysts for enantioselective hydrogenation

Article abstract of DOI:10.1016/j.catcom.2011.08.006

Metallic Ni catalysts co-modified with (R,R)-tartaric acid and NaBr showed high enantioselectivity and durability upon hydrogenation of methyl acetoacetate to give methyl 3-hydroxybutyrate. The chirally modified catalyst prepared from 3-μm Ni powder was highly robust to maintain the hydrogenation activity and enantiodifferentiating ability for ca. 3 months under dry condition, which enables long-term storage and hence facilitates commercial distribution and industrial application.

Full text of DOI:10.1016/j.catcom.2011.08.006

Catalysis Communications 15 (2011) 15–17
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Durability enhancement of chirally modified metallic nickel catalysts for
enantioselective hydrogenation
a
a
b
b
Tsutomu Osawa a, , Tomoko Kizawa , I-Yin Sandy Lee , Shinji Ikeda , Takayuki Kitamura ,
⁎
Yoshihisa Inoue c, Victor Borovkov b,c,⁎⁎
Graduate School of Science and Engineering for Research, University of Toyama, Gofuku, Toyama 930-8555, Japan
Metek Kitamura Co., Ltd., 1 Warada-cho, Kamitoba, Minami-ku, Kyoto 601–8133, Japan
Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2011
Received in revised form 14 July 2011
Accepted 1 August 2011
Available online 8 August 2011
Metallic Ni catalysts co-modified with (R,R)-tartaric acid and NaBr showed high enantioselectivity and
durability upon hydrogenation of methyl acetoacetate to give methyl 3-hydroxybutyrate. The chirally
modified catalyst prepared from 3-μm Ni powder was highly robust to maintain the hydrogenation activity
and enantiodifferentiating ability for ca. 3 months under dry condition, which enables long-term storage and
hence facilitates commercial distribution and industrial application.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Modified metallic nickel
Enantioselective hydrogenation
Durability
Methyl acetoacetate
Tartaric acid
1. Introduction
consideration: (1) the enantio-selectivity upon hydrogenation; (2)
the durability upon repeated use; (3) the shelf life of activity and
Developing facile method for preparing optically pure compounds is
one of the most important and challenging tasks in current synthetic
chemistry. Amongst various chemical and biochemical methods to
prepare optically active compounds, the use of heterogeneous chiral
catalysts is promising in particular for large-scale production in industry
[1]. This type of catalysts can be readily prepared, recovered and reused,
yet less expensive and ecologically benign. For enantio-differentiating
heterogeneous hydrogenation, two major catalytic systems, i.e. alkaloid-
modified precious metals [2–4] and tartaric acid (TA)-modified nickels
[5–8], are known to give high enantio-selectivities. Thus, alkaloid-
modified platinum catalysts are used for the hydrogenation of prochiral
α-ketoesters, while alkaloid-modified palladium catalysts for prochiral
alkenes. On the other hand, nickel catalysts co-modified with TA
and NaBr are successful in hydrogenating prochiral ketones, such
as β-ketoesters and 2-alkanones, to the corresponding chiral
alcohols in high enantio-purities.
selectivity. However, apart from the enantio-selectivity, the durability
and shelf life have rarely been investigated in detail. For example, in the
case of alkaloid-modified precious metal catalysts, the cinchonidine
(commonly used as the most effective modifier) adsorbed on platinum
is liable to leach out from the catalyst surface upon repeated use, and
hence the chiral modifier has to be added after each hydrogenation run
to keep the original enantio-selectivity [9]. In contrast, the durability of
TA-modified nickel catalyst can be greatly improved by embedding the
catalyst in silicone polymer, while retaining the 75% enantio-selectivity
for repeated 27 runs [10]. More recently, we demonstrated that in situ
TA/NaBr modification of metallic nickel prepared by reducing nickel
oxide leads to a durable nickel catalyst that maintains the enantio-
selectivity of 80% for 30 catalytic cycles [11]. Chen et al. also reported
that TA-modified Raney nickel catalyst keeps the enantio-selectivity of
N60% even after 11 repeated runs, provided that a small amount of NaBr
is added after each catalytic cycle [12].
In applying a heterogeneous catalyst to practical hydrogenation
reactions, there are several important factors to be taken into
Nickel is much less expensive than platinum or palladium, but has
an inherent drawback that its metallic surface is liable to be oxidized
upon exposure to air. It is believed therefore that the chirally modified
nickel catalyst should be used immediately after the preparation to
achieve the optimal activity and enantio-selectivity. Consequently, little
is known about the shelf life of TA-modified catalyst. Nevertheless,
Sugimura et al. reported that 1,2,4-butanetriol as an additive enables
longer storage of the cinchonidine modified palladium catalyst under
nitrogen atmosphere [13]. Tai et al. also have shown that TA-modified
⁎
Corresponding author. Tel.: +81 76 445 6611; fax: +81 76 445 6549.
⁎⁎ Correspondence to: V. Borovkov, Department of Applied Chemistry, Osaka
University, 2-1 Yamada-oka, Suita 565-0871, Japan. Tel.: +81 6 6879 4128; fax: +81
6 6879 7923.
E-mail addresses: osawa@sci.u-toyama.ac.jp (T. Osawa),
victrb@chem.eng.osaka-u.ac.jp (V. Borovkov).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.08.006

16
T. Osawa et al. / Catalysis Communications 15 (2011) 15–17
nickel embedded in silicone polymer can be stored in the air for
6 months without any accompanying decline of the enantio-selectivity
upon hydrogenation [10]. However, the siliconepolymer easily swells to
absorb part of the reaction mixture under the hydrogenation conditions
(at 373 K and 9 MPa of hydrogen), and the absorbed product is difficult
to retrieve from the swelled silicone. This reveals the limitation of such a
strategy to extend the shelf life and a more straightforward method to
prepare a durable catalyst of long shelf life is highly desired.
In the present study to obtain a highly active and enantio-
differentiating, yet durable, chirally modified nickel catalyst, we pre-
pared the TA-modified nickel catalysts not only from Raney nickel alloy
but also from fine nickel powder and compared their performance upon
hydrogenation of methyl acetoacetate after varying periods of storage
under wet and dry conditions (Scheme 1).
the hydrogenation product determined by polarimetry; i.e., e.d.
a=optical purity (%)=100×([α]D20 of methyl 3-hydroxybutyrate
obtained by hydrogenation)/([α]D20 of optically pure enantiomer),
where the specific rotation ([α]D20) of optically pure (R)-methyl 3-
hydroxybutyrate is −22.95º (neat) [14]. When the conversion of the
methyl 3-hydroxybutyrate was not 100%, the e.d.a. was evaluated by
using gas-liquid chromatography (GLC). Acetylation of the sample was
carried out using acetyl chloride and pyridine. A portion of the
acetylated sample was subjected to the analysis using a chiral capillary
gas chromatograph equipped with CP Chirasil DEX-CB (0.25 mm×25 m,
column temperature: 363 K). The e.d.a was calculated from the peak
integration of the corresponding enantiomers.
3. Results and discussion
2. Experimental
The effects of storage period on the hydrogenation activity and
enantio-differentiating ability (e.d.a.) of the catalysts were examined by
using the TA-modified Raney nickel and 3-μm nickel powder catalysts
stored for 0–21 days under the wet condition at ambient temperature.
As can be seen from Table 1, quantitative conversions were
achieved even after 7 and 21 days of storage with the modified Raney
nickel and modified 3-μm nickel powder, respectively, indicating the
hydrogenation activity is kept at least for 1–3 weeks under the wet
condition. However, the e.d.a. of the modified Raney nickel catalyst
showed a sudden drop from 75% to 36% after 1 day and was kept low
thereafter, as anticipated from the previous research [10,15–17]. In
contrast, the e.d.a. of the modified 3-μm nickel powder catalyst was
much more persistent for a longer period of time, exhibiting only slow
decrease from 84% to 79% after 1 day, then to 75% after 1 week, and
eventually to 69% even after 3 weeks of storage. The longer shelf life of
the nickel powder catalyst, compared to the modified Raney nickel
catalyst, may be attributed to the absent of aluminum remnant, which
is readily oxidized under the condition employed (vide infra), causing
the overall reduction of the enantioselectivity. The smaller, but
noticeable, decrease of e.d.a. observed for 3-μm nickel powder catalyst
may be ascribed to the slower oxidation and the subsequent structural
changes of the nickel surface under the wet condition (vide infra).
These results prompted us to further examine the effect of drying
catalyst on the e.d.a. The modified Raney nickel and nickel power
catalysts were dried at different temperatures of 323 K and 353 K, and
the results of hydrogenation were compared with those obtained with
the freshly prepared catalysts (Table 2).
It is somewhat unexpected that both the catalysts kept the original
hydrogenation activity even after drying for 18.5 h at 353 K to give the
quantitative conversions. However, the e.d.a. was much more susceptive
to drying in particular for the Raney nickel catalyst to give a significantly
reduced e.d.a. of 53% upon drying at 353 K. In sharp contrast, the nickel
powder catalyst was more robust to maintain the originally high e.d.a.
even after drying. Thus, the e.d.a. obtained with the nickel powder
catalyst dried at 323 K or 353 K showed only a slight decrease from 84%
to 82% or to 78%, respectively. These results reveal the great advantages
of the chirally modified nickel powder catalyst over the Raney nickel
catalyst in all aspects of hydrogenation activity, e.d.a., shelf life, and
durability.
For the preparation of modified Raney nickel catalyst, 1.24 g of
Raney nickel alloy (Ni:Al=41:59) (Kawaken Fine Chemicals, Saitama,
Japan) was digested for 1 h in 13 cm3 of 20% aqueous NaOH solution
at 373 K. After removal of the alkaline solution by decantation, the
catalyst was washed 15 times with 15-cm3 portions of deionized
water. For the activation of metallic nickel powder (Aldrich, 3 μm in
diameter), 0.5 g of nickel powder was treated for 0.5 h in a hydrogen
stream at 473 K. The activated nickel catalyst, i.e. Raney nickel or
nickel powder, was immersed in a 50-cm3 aqueous solution contain-
ing (R,R)-tartaric acid (0.5 g) and NaBr (0.5 g for Raney nickel and
0.01 g for nickel powder) at 373 K for 1 h. The pH of the modifier
solution was adjusted in advance to 3.2 with a 1 M aqueous NaOH
solution. For the hydrogenation reaction immediately after the
modification (without any storing), the solution was removed by
decantation and the catalyst was successively washed once with
10 cm3 of deionized water, twice with 25 cm3 of methanol, and then
twice with 10 cm3 of tetrahydrofuran (THF). For the hydrogenation
reaction after the storage for a given period of time under the wet
condition, the modifying solution was removed by decantation and
the catalyst was washed twice with 10 cm3 of deionized water, and
stored under nitrogen atmosphere in a minimum amount of deionized
water to cover the catalyst. Before use the catalyst was washed twice
with 25 cm3 of methanol and then twice with 10 cm3 of THF. For the
hydrogenation reaction after the storage under the dry condition, the
modifying solution was removed by decantation and the catalyst was
successively washed once with 10 cm3 of deionized water and twice
with 25 cm3 of ethanol. The washed catalyst was then dried under
vacuum (4.0 kPa) for 18.5 h and stored under nitrogen in a glass vial.
The stored dry catalyst was used for the hydrogenation reaction
without any further treatment.
By using the chirally modified catalyst prepared from 1.24 g of Raney
nickel alloy or prepared from 0.5 g of nickel powder, the hydrogenation
reaction was carried out. The hydrogenation of methyl acetoacetate
(5 g) dissolved in a mixture of acetic acid (0.1 g) and THF (10 cm3) was
run for 20 h at 373 K and the initial hydrogen pressure of 9 MPa in an
autoclave equipped with a magnetically coupled mechanical stirrer.
The hydrogenation product, methyl 3-hydroxybutyrate, was isolated
from the reaction mixture by distillation. The enantio-differentiating
ability (e.d.a.) of the catalyst was evaluated by the optical purity of
Since the modified 3-μm nickel powder catalyst dried at 323 K
showed almost the same e.d.a. as the freshly modified catalyst, the effects
Scheme 1. Enantio-differentiating hydrogenation of methyl acetoacetate.

T. Osawa et al. / Catalysis Communications 15 (2011) 15–17
17
Table 1
Effects of the storage period on the conversion and e.d.a. under wet condition.
Table 3
Effects of the storage period on the conversion and e.d.a. under dry conditiona.
Storage period/day
Modified Raney nickel
Modified 3-μm nickel
Storage period/day
Modified Raney nickel
Modified 3-μm nickel
Conv./%
E.d.a./%
Conv./%
E.d.a./%
Conv./%
E.d.a./%
Conv./%
E.d.a./%
0a
1
7
14
21
100
100
100
–
75
36
36
–
100
100
100
99
84
79
75
64
69
0b
1
7
21
84
100
100
100
–
75
63
61
–
100
100
100
100
82
84
82
81
82
79
–
–
99
–
–
a
a
Hydrogenation was carried out just after the modification.
Dried at 323 K for 18.5 h.
The catalyst was kept wet and used immediately after the modification.
b
observed for the modified 3-μm nickel powder catalyst stored under the
dry condition is considered to arise from the oxidation of nickel surface.
In conclusion, the TA-modified nickel catalyst prepared from 3-μm
nickel powder showed a remarkably high e.d.a. even after 84 days of
storage under the dry condition. Metallic nickel powder was shown to be
a promising starting material for modified nickel catalysts for asymmetric
hydrogenation, which are active, efficient, durable, yet inexpensive.
Table 2
Effects of the drying temperature on the conversion and e.d.a.a
Drying temperature/K
Modified Raney nickel
Modified 3-μm nickel
Conv./%
E.d.a./%
Conv./%
E.d.a./%
b
100
100
100
75
63
53
100
100
100
84
82
78
323
353
a
Dried for 18.5 h at the designated temperature.
The catalyst was kept wet and used immediately after the modification.
Acknowledgements
b
This work was supported by a grant-in-aid (No. P215260133) from
Japan National Federation of Small Business Association, which is
gratefully appreciated. YI thanks the Japan Society for the Promotion
of Science for supporting this project through Grant-in-Aid (A) No.
21245011.
of storage period on its catalyst activity and e.d.a. were examined to give
the results shown in Table 3.
The e.d.a. of the dried Raney nickel-based catalyst deteriorated from
75% to 63% only after 1 day, whilst the dried nickel powder-based
catalyst nicely maintained the high e.d.a. of 82% after 21 days and of 79%
even after 84 days of storage. These results clearly demonstrate that
both of the chirally modified Raney and the powder nickel catalysts
maintain the original e.d.a. for a longer period of time under the dry
rather than wet condition. In particular, the TA-modified nickel powder
is well suited for commercial purposes in view of the high e.d.a. and the
facile storage and distribution.
The Raney nickel alloy (Ni:Al=41:59 w/w) consists of NiAl3, Ni2Al3,
and the Al/NiAl3 eutetic. The initial treatment with 20% NaOH solution
does not fully dissolve the aluminum component from these phases, as
the existence of aluminum is detected in the activated Raney nickel
catalyst [18]. The subsequent chiral modification in a pH 3.2 solution at
373 K further dissolves the residual aluminum, but a certain amount of
aluminum still remains in the modified Raney nickel catalyst [14]. The
noticeable decrease of e.d.a. observed for the modified Raney nickel
upon storage could be attributed to the oxidation of catalyst surface
under the wet and dry conditions and/or the dissolution of aluminum
into the remaining water under the wet condition. Furthermore, the
possible change on the catalyst surface is likely to alter the spatial
orientation of the TA adsorbed on the nickel surface, thus reducing the
enantio-selectivity upon hydrogenation. The better e.d.a. obtained with
such modified catalysts that were stored under dry, rather than wet,
condition support this rationalization. Thus, the small decrease in e.d.a.
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