Simplified preparation of chirally modified nickel catalyst for enantioselective hydrogenation: A step forward to industrial use

Article abstract of DOI:10.1016/j.apcata.2012.08.035

A chirally modified nickel catalyst for the enantio-differentiating hydrogenation of β-ketoesters is conventionally prepared by immersing the pre-activated metallic nickel into an aqueous solution of enantiopure tartaric acid (so called "modification step"). During the pre-activation step, nickel precursor is commonly treated with hydrogen gas at elevated temperatures of up to 473 K. The X-ray photoelectron spectral examinations of chirally modified nickel catalysts obtained under the different modification conditions revealed that the chiral modification process itself plays a major role in activating the nickel surface whilst the pre-activation procedure is a less important factor. The corresponding enantio-differentiating hydrogenations of methyl acetoacetate in the liquid phase using the prepared chiral catalysts unambiguously confirmed this conclusion, providing quantitative conversions and high enantioselectivities of up to 90%.

Full text of DOI:10.1016/j.apcata.2012.08.035

Applied Catalysis A: General 445–446 (2012) 269–273
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Simplified preparation of chirally modified nickel catalyst for enantioselective
hydrogenation: A step forward to industrial use
Tsutomu Osawaa,∗, I-Yin Sandy Leea, Shinji Ikedab, Takayuki Kitamurab, Yoshihisa Inouec,
Victor Borovkovb,c,∗∗
a Graduate School of Science and Engineering for Research, University of Toyama, Gofuku, Toyama 930-8555, Japan
b Metek Kitamura Co. Ltd., 1 Warada-cho, Kamitoba, Minami-ku, Kyoto 601-8133, Japan
c Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 May 2012
Received in revised form 21 August 2012
Accepted 25 August 2012
Available online 1 September 2012
A chirally modified nickel catalyst for the enantio-differentiating hydrogenation of ␤-ketoesters is
conventionally prepared by immersing the pre-activated metallic nickel into an aqueous solution of
enantiopure tartaric acid (so called “modification step”). During the pre-activation step, nickel pre-
cursor is commonly treated with hydrogen gas at elevated temperatures of up to 473 K. The X-ray
photoelectron spectral examinations of chirally modified nickel catalysts obtained under the different
modification conditions revealed that the chiral modification process itself plays a major role in activat-
ing the nickel surface whilst the pre-activation procedure is a less important factor. The corresponding
enantio-differentiating hydrogenations of methyl acetoacetate in the liquid phase using the prepared
chiral catalysts unambiguously confirmed this conclusion, providing quantitative conversions and high
enantioselectivities of up to 90%.
Keywords:
Chirally modified metallic nickel
Enantioselective hydrogenation
Hydrogen activation
XPS
© 2012 Elsevier B.V. All rights reserved.
Methyl acetoacetate
Tartaric acid
1. Introduction
palladium catalysts are applied for the hydrogenation of alkene
derivatives (90–94% ee) [18–21]. On the other hand, the nickel cat-
Developing a facile method for preparing optically pure com-
pounds is one of the most important yet challenging tasks of current
synthetic chemistry. Amongst various chemical and biochemical
procedures for manufacturing optically active compounds, the use
of heterogeneous chiral catalysts is a very promising approach in
particular for the large-scale industrial production [1]. This type
of catalysts can be readily prepared, recovered, and reused, yet is
less expensive and ecologically benign. Furthermore, the desired
products prepared by such catalytic processes are easily separable
from the reaction mixture. In the case of enantio-differentiating
heterogeneous hydrogenations, two catalytic systems, i.e., cin-
chona alkaloid-modified precious metals [2–4] and tartaric acid
(TA)-modified nickels [5–8], are known to be the most effi-
cient types of catalysts giving high enantioselectivities for specific
prochiral substrates. Thus, the alkaloid based platinum catalysts
are commonly used for the hydrogenation of ␣-ketoesters and
other activated ketones (95–98% ee) [9–17], whilst the modified
alysts co-modified with TA and NaBr are known to be effective in
hydrogenating various prochiral ketones, such as ␤-ketoesters and
2-alkanones, yielding the corresponding chiral alcohols with the
enantiomeric excess of up to 85% for 2-alkanones [22] and 98% for
␤-ketoesters [23].
Whilst the types of prochiral substrates are different for
the above-mentioned catalytic systems, the procedures used for
preparing these catalysts are also distinguishable. For example,
the alkaloid modified precious metal catalysts are generally pre-
pared in situ by adding a chiral modifier directly into the reaction
medium. Although the in situ modification can also be applied for
the preparation of tartaric acid-modified nickel catalyst [24], the
chiral catalyst obtained by this method shows a rather modest per-
formance. Therefore, the modification procedure is usually carried
out prior to the hydrogenation reaction typically by immersing the
activated nickel precursor into a solution of TA [25–27] or TA and
NaBr (so-called a conventional modification) [27–31]. By employ-
ing the conventional modification method, we aim to expand the
range of nickel sources suitable for the heterogeneous hydrogena-
tion.
∗
Corresponding author. Tel.: +81 76 445 6611; fax: +81 76 445 6549.
Corresponding author at: Department of Applied Chemistry, Osaka University,
∗∗
It has been postulated that the modification of nickel with TA
and NaBr plays two major roles, i.e., (i) the adsorption of TA and
NaBr on the surface of Ni and (ii) the formation of appropriate
2-1 Yamada-oka, Suita 565-0871, Japan. Tel.: +81 66 879 4128; fax: +81 66 879 7923.
E-mail addresses: osawa@sci.u-toyama.ac.jp (T. Osawa),
victrb@chem.eng.osaka-u.ac.jp (V. Borovkov).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.08.035

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T. Osawa et al. / Applied Catalysis A: General 445–446 (2012) 269–273
Scheme 1. Enantio-differentiating hydrogenation of methyl acetoacetate.
surface for the enantio-differentiation (by smoothing the defects
on the surface and chirally modifying the smoothen surface with
TA) [32]. In order to understand how the changes of nickel surface
upon modification affect the process of enantio-differentiation, a
number of X-ray photoelectron spectroscopic (XPS) studies have
been carried out by several research groups to date. For exam-
ple, the adsorption mode of modifier [33], the mechanisms of
enantio-differentiation on the tartaric acid-modified mixed nickel
and cerium oxides [34], on the tartaric acid-modified Raney nickel
[35], and on the (S)-aspartic acid modified nickel foil [36] were
investigated. However, a rather limited number of XPS studies on
the chirally modified nickel catalysts make it difficult to produce an
adequate rationale concerning the mechanistic aspects of enantio-
differentiating hydrogenation.
In the present study, we aim to fill this gap by studying the chi-
rally modified catalysts on the basis of metallic nickel powders with
and without the pre-activation procedure by a stream of hydrogen
treated with an aqueous solution of TA (in the presence and absence
of NaBr). During the course of catalyst’s preparation, the activation
of nickel surface to obtain Ni0 domains on the surface is consid-
ered to be an important factor, because the effective dissociation of
hydrogen molecules to achieve the high hydrogenation activity is
performed on the Ni0 species [37]. Therefore, the effects of hydro-
gen pre-activation and subsequent modification, which control the
fraction of Ni0 species on the nickel surface, on the hydrogenation
efficiency are of prime interest. Furthermore, the risk-free method
of surface activation is important for the large-scale industrial pro-
duction of modified nickel catalyst. For example, the potentially
hazardous activation of nickel surface by hydrogen gas at the ele-
vated temperature is not appropriate for the industrial application.
Therefore, the roles of hydrogen activation and subsequent mod-
ification were comprehensively investigated by XPS and the data
obtained were analyzed in conjunction with the results of enantio-
differentiating hydrogenation of methyl acetoacetate (Scheme 1)
obtained by using the same modified nickel catalyst with and with-
out hydrogen treatment.
procedure after the modification was the same as that employed
for the hydrogen-activated nickel.
2.2. Measurement of XP spectra
XPS was conducted on an ULVAC-PHI ESCA 5800 with Mg K␣
as a radiation source. All measurements were made at room tem-
perature. The samples for the measurements were prepared under
air and at ambient temperature and then introduced to the cham-
ber. XP spectra were processed using the program CasaXPS version
2.3.1.15. The carbon (1s) line was calibrated at 284.8 eV. For the
measurement of XP spectra, the washed sample after the modifica-
tion mentioned above was dried under vacuum (4.0 kPa) at 323 K
for 18.5 h and stored under argon in a sealed glass ampoule. The
stored dry catalyst was used for measurement immediately after
opening the ampoule.
2.3. Enantio-differentiating hydrogenation with modified nickel
catalyst
The modified catalyst was placed into an autoclave equipped
with a magnetically coupled mechanical stirrer and contained a
mixture of methyl acetoacetate (5 g), acetic acid (0.1 g), and THF
(10 cm3). It is of note that acetic acid was added for increasing
the enantioselectivity by accelerating the hydrogenation rate of the
enantio-differentiating sites [38]. The hydrogenation reaction was
run for 20 h at 373 K and an initial hydrogen pressure of 9 MPa. The
hydrogenation product, methyl 3-hydroxybutyrate, was isolated
from the reaction mixture by distillation. The enantioselectivity
was measured by using gas–liquid chromatography (GLC). Acet-
ylation of the reaction product for the GLC measurements was
carried out by using acetyl chloride and pyridine. A portion of the
acetylated sample was subjected to the chiral GLC analysis on a
CP Chirasil DEX-CB column (0.25 mm × 25 m) operated at 363 K.
The enantioselectivity was calculated from the peak integration
of the corresponding enantiomer peaks. The reproducibility of the
enantioselectivity was found to be within 2%.
2. Experimental
2.1. Modification of metallic nickel
3. Results and discussion
Metallic nickel powders (Aldrich, 3 ␮m in diameter and Nano
Clean Science Co. Ltd., 5 ␮m in diameter) were used as a source of
nickel catalyst. For the hydrogen activation, 0.5 g of nickel powder
was treated for 0.5 h in a stream of hydrogen at 473 K. For the chiral
modification, the pre-activated nickel powder was immersed in a
50-cm3 aqueous solution containing (R,R)-tartaric acid (0.5 g) (and
NaBr (0.5 g) in some cases) at 373 K for 1 h. The pH value of modifi-
cation solution was adjusted in advance with a 1 M aqueous NaOH
solution to maintain 3.2 or 5.0. The pH of modification solution sig-
nificantly affects the enantioselectivity and pH 3.2 was found to
be optimal for the reaction conditions used [8]. The modification
solution was removed by decantation and the catalyst was suc-
cessively 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 chiral modification without the hydrogen activation,
the nickel powder was used as received and directly immersed
into the above-mentioned modification solution. The washing
3.1. XPS analysis of modified nickel catalysts
The effects of hydrogen treatment and chiral modification on the
catalyst’s surface composition of nickel species were firstly exam-
ined by XPS measurements by using the 3 ␮m Ni powder. Whilst
all the expected atomic components of the analyzed samples were
detected by XPS, the relative area of each nickel species is of our par-
ticular interest in relation to the catalytic properties, as mentioned
above. Thus, Fig. 1 shows the XP spectra of Ni2p3/2 and Ni2p1/2
regions.
The peak at 852.4 0.2 eV is attributed to the Ni0 binding energy,
whilst the peaks at 854.6 eV and 856.1 eV are assigned to the Ni2+
and Ni3+ states, respectively [39]. The peaks around 859 eV and
861 eV are known to be the satellites of Ni0 and Ni2+, correspond-
ingly [40], as is the case with the peaks at 877 eV and 880 eV. The
doublet separation of Ni2p3/2 and Ni2p1/2 of Ni0 was set to be
17.2 eV and that of Ni2+ to be 18.0 eV [40]. The deconvolution of the

T. Osawa et al. / Applied Catalysis A: General 445–446 (2012) 269–273
271
areas of Ni2p3/2 and Ni2p1/2 was simultaneously carried out. The
composition of Ni0 was calculated by using the peaks of Ni2p3/2
.
Table 1 shows the effects of hydrogen treatment and subsequent
chiral modifications (with TA only or simultaneously with TA and
NaBr) on the composition of nickel species. The original metallic
3 ␮m nickel powder as received, free from the hydrogen treatment
and chiral modification, showed 9% Ni0 and 91% Ni2++Ni3+ (entry 1),
thus clearly indicating that the surface is predominantly oxidized.
In contrast, the nickel powder, which was treated with hydrogen
and subsequently modified with a solution containing only TA (the
condition used for the conventional catalyst activation and modi-
fication [41]), contained 46% Ni0 and 54% Ni2+ + Ni3+ on the surface
(entry 2). In this case, the ratio of Ni0 to Ni2+ + Ni3+ is close to
1:1, which is comparable to the result of Kukula et al. reported
for the chirally modified Raney nickel catalyst [35]. However, sur-
prisingly, the TA-modified nickel powder without the hydrogen
pre-activation also showed very similar results: 45% Ni0 species
and 55% Ni2+ + Ni3+ (entry 3). The comparable Ni0 contents for the
nickel powder with and without hydrogen treatment indicate that
the modification with a solution of tartaric acid at pH 3.2 is able
to effectively activate the nickel surface by removing the domains
of nickel oxide from the surface even without the pre-treatment
procedure. To gain a further confirmation of the surface activation
during the modification process, a different condition of surface
treatment were applied by using the 5 ␮m Ni powder. Hence, upon
using the 1 M TA solution at 323–328 K without the H2 treatment
and pH adjustment, the corresponding Ni0/(Ni2+ + Ni3+) ratio was
found to be 8%/92%, whilst in the case of original (as received) nickel
powder the corresponding ratio is 0%/100%. These results also indi-
cate clearly that the modification with TA solution is able to activate
the nickel surface. However, the small fraction of Ni0 in the con-
centrated conditions used is apparently due to the considerable
formation of Ni tartrate on the surface.
The effect of air exposure (before XPS analysis) on the state of Ni0
was separately examined by using the same 5 ␮m Ni powder. Thus,
the corresponding percentage areas of Ni0/Ni2+ + Ni3+ without H2
treatment and with H2 treatment (473 K, 0.5 h) were found to be
significantly different: 0%/100% and 28%/72%, respectively. These
data clearly indicate that whilst the original Ni powder is fully in
the oxidized state, the subsequent reduction with hydrogen leads to
the formation of a substantial amount of Ni0, which is a prerequisite
for the catalytic hydrogenation.
On the basis of above results, it is reasonable to conclude that
the conventional modification process plays three key roles in the
enantio-selective hydrogenation; (i) the activation of nickel sur-
face (by enhancing the Ni0 content), (ii) the adsorption of modifier
on the surface, and (iii) the generation of the appropriate surface
structure suitable for the enantio-differentiation. Although the last
two aspects are indispensable for enantio-differentiating catalytic
hydrogenation and therefore have been discussed previously [32],
the first one is essentially a new finding. Conventionally, the pre-
treatment of nickel with hydrogen is believed to be absolutely
necessary in order to activate the nickel surface before the chi-
ral modification. However, the present result clearly indicates that
the surface activation also proceeds in situ upon the chiral modi-
fication with an acidic TA solution. This finding, which enables the
direct use of commercially available nickel powders, is particularly
important in view of the industrial application of chirally modified
nickel catalysts.
In general, tartaric acid is adsorbed as nickel sodium tartrate
when the chiral modification is carried out at pH 3.2 whilst increas-
ing the pH above 5 shifts the adsorption species towards disodium
tartrate [42]. Since the modification pH was set at 3.2 in the present
study, it is expected that the adsorbed tartaric acid should be
in the form of nickel sodium tartrate. Therefore, the total area
of Ni2+ + Ni3+ determined by XPS is considered to consist of two
Fig. 1. XP spectra of the Ni area of metallic nickel powder under the following con-
ditions: (a) no hydrogen treatment and no subsequent chiral modification (entry no.
1), (b) hydrogen treatment and subsequent chiral modification with TA only (entry
no. 2), (c) no hydrogen treatment but with chiral modification with TA only (entry
no. 3), (d) hydrogen treatment and subsequent chiral modification with TA and NaBr
(entry no. 4), (e) no hydrogen treatment but with chiral modification with TA and
NaBr (entry no. 5).

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T. Osawa et al. / Applied Catalysis A: General 445–446 (2012) 269–273
Table 1
Effects of the pre-activation by hydrogen and the chiral modification on the composition of nickel species determined by XPS analysis.
Catalyst entry
Pre-activation by hydrogen
Modifier
Ni0 (%)
Ni2+ (%)
Ni3+ (%)
Ni2+ + Ni3+ (%)
1
2
3
4
5
None
473 K, 0.5 h
None
473 K, 0.5 h
None
None
TA 0.5 g
TA 0.5 g
TA 0.5 g, NaBr 0.5 g
TA 0.5 g, NaBr 0.5 g
9
46
45
41
39
26
26
27
28
30
65
28
28
31
31
91
54
55
59
61
Table 2
Effects of the pre-activation by hydrogen and chiral modification on the enantio-differentiating hydrogenation of methyl acetoacetate.
Catalyst entry
Pre-activation by
hydrogen
Modifier
Modification pH
Conversion (%)
Enantiomeric
excess (%)
1
2
3
4
5
6
7
None
473 K, 0.5 h
None
473 K, 0.5 h
None
473 K, 0.5 h
None
None
TA 0.5 g
TA 0.5 g
TA 0.5 g, NaBr 0.5 g
TA 0.5 g, NaBr 0.5 g
TA 0.5 g, NaBr 0.5 g
TA 0.5 g, NaBr 0.5 g
–
67
100
99
–
82
80
90
86
86
80
3.2
3.2
3.2
3.2
5.0
5.0
99
100
100
100
The reproducibility of the enantiomeric excess was within 2%.
distinct contributions: one from the area covered by nickel sodium
tartrate and the other from nickel oxide remaining even after the
acidic TA modification.
(in the case of entries 6 and 7), may be attributed to a little
more efficient removal of nickel oxide from the surface upon the
hydrogen pre-treatment. It is known that the addition of NaBr
enhances the enantioselectivity of TA-modified nickel catalysts by
blocking the non enantio-differentiating sites on the nickel sur-
face [43]. Therefore, the addition of NaBr (entries 4 and 5) as
expected attained higher enantioselectivities than in the case of
modification with only TA (entries 2 and 3), irrespective of some-
what smaller Ni0 area of the catalysts modified with TA and NaBr.
The observed pH effect is in full agreement with the previously
reported data on the highest enantioselectivity found for the cat-
alyst prepared at pH 3.2 and with the hydrogen pre-treatment
[8]. Similar results were also obtained without hydrogen pre-
treatment (see, entries 5 and 7). The fact that the chirally modified
nickel catalyst without hydrogen activation can give the ee value
as high as 86% is advantageous in particular for the large-scale
production and application of this modified catalyst in industry,
since the activation process with hydrogen gas could be elimi-
nated.
In conclusion, the XPS studies of tartaric acid-(NaBr-)modified
nickel catalysts with and without pre-activation by hydrogen gas
(prior to the modification) revealed that almost the same surface
contents of Ni0 were attained even if commercially available nickel
powders were used as received. More importantly, even without
the hydrogen activation, almost the same enantioselectivity was
attained in the enantio-differentiating hydrogenation of methyl
acetoacetate with modified nickel catalysts. These results clearly
indicate that the chiral modification process plays the third cru-
cial role of simultaneous in situ surface activation during the chiral
modification besides the two well-known roles, i.e., the adsorption
of tartaric acid on the nickel surface and the formation of appropri-
ate smooth surface for the enantio-differentiation by etching. This
finding also facilitates the large-scale production and application
to asymmetric synthesis of the chirally modified nickel catalyst in
industry, as the potentially hazardous pre-activation by hydrogen
gas at elevated temperatures could be avoided without any serious
loss of enantioselectivity.
When nickel powder was modified with an aqueous solution
containing TA and NaBr, the amount of Ni0 on the surface was
decreased from 45–46% (the values obtained for the TA-modified
nickel) down to 39–41%, irrespective of the presence/absence of
the pre-activation (entries 4 and 5). This would be attributed to the
less corrosive activity of the solution of TA and NaBr than that of
TA only, i.e., more effective removal of nickel oxide by the solution
of TA without NaBr.
3.2. Enantio-differentiating hydrogenation with modified nickel
catalyst
In order to understand the correlation between the surface
nickel composition and the asymmetric catalytic activity, the
enantio-differentiating hydrogenation of methyl acetoacetate was
carried out as a benchmark reaction for evaluating the nickel cat-
alysts studied by XPS (Scheme 1). Table 2 shows the effects of
hydrogen treatment and chiral modification on the conversion and
the enantioselectivity in these catalytic reactions.
Except the original (as received) Ni powder (entry 1), the effi-
ciency of hydrogenation was equally high in all the cases to
achieve almost 100% conversion after 20 h, irrespective of the pre-
activation and modification conditions. Although the conversion
value of original Ni powder was low, the comparable hydrogena-
tion activities and enantio-selectivities obtained for the chirally
modified catalysts with and without pre-activation clearly indicate
that the in situ surface activation proceeds simultaneously during
the chiral modification, which is in nice agreement with the XPS
observation described above.
A further support for this conclusion was obtained from the
hydrogenation experiments using nickel catalyst chirally modified
by TA and NaBr. The same effect was also found in this study.
When the modification pH was fixed to be 3.2, the pre-activated
nickel powder modified with TA and NaBr gave the hydrogena-
tion product in 90% ee (entry 4), whilst the similarly modified
catalyst without the pre-activation also afforded the product in
slightly reduced 86% ee (entry 5). However, increasing the pH up
to 5.0 resulted in decreasing the ee values down to 86% (entry
6) and 80% (entry 7), respectively. The difference in the ee val-
ues, which is close to the experimental error ( 2%) (in the case
of entries 2 and 3, entries 4 and 5, respectively) or slightly higher
Acknowledgements
This work was supported by Adaptable and Seamless Tech-
nology Transfer Program through target-driven R&D, JST (No.
AS2311578D) and Grant-in-Aid for Scientific Research from JSPS
(No. 21245011), both of which are gratefully acknowledged.

T. Osawa et al. / Applied Catalysis A: General 445–446 (2012) 269–273
273
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