Effects of the preparation method on the performance of the Cu/ZnO/Al 2O3 catalyst for the manufacture of l-phenylalaninol with high ee selectivity from l-phenylalanine methyl ester

Article abstract of DOI:10.1039/c3cy00937h

The effects of the preparation method on the properties of Cu/ZnO/Al ₂O₃ catalysts for l-phenylalanine methyl ester hydrogenation to l-phenylalaninol were investigated in detail, including the precipitation method and conditions (the aging time, calcination temperature and so on), with the help of ICP-OES, N₂ and N₂O adsorption, XRD, H₂-TPR and TEM techniques. The results show that physicochemical properties of the catalysts are greatly affected by the preparation method and conditions. The uniform size distribution of CuO species can be obtained by fractional co-precipitation. The appropriate aging time is 2 h, and the catalyst aged for 2 h has the largest metallic copper surface area (S_Cu) and surface copper amount and the smallest CuO crystallites. The lower calcination temperature is favorable for increasing the surface area and metallic copper surface area of the catalyst. The spinel structure CuAl₂O₄ phase can form after calcination at 550 °C. The turnover frequency (TOF) values of l-phenylalaninol formed using different catalysts indicate the structurally sensitive character of the title reaction, and S_Cu is not the sole cause affecting the catalytic activities of the catalysts. B-TOF on the basis of the active sites (Cu⁰) in the boundary between CuO and ZnO or Al₂O₃ was proposed; the relationships of B-TOF with d_CuO (particle size of CuO) and S_Cu were established. Using the Cu/ZnO/Al₂O₃ catalyst prepared by fractional co-precipitation with aging at 70 °C for 2 h and calcination at 450 °C for 4 h, 83.6% selectivity to l-phenylalaninol without racemization was achieved. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy00937h

Catalysis
Science &
Technology
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PAPER
Effects of the preparation method on the
performance of the Cu/ZnO/Al O catalyst for the
2
3
Cite this: Catal. Sci. Technol., 2014,
manufacture of L-phenylalaninol with high ee
selectivity from L-phenylalanine methyl ester
4, 1132
a
a
a
ab
Zhangping Shi, Xiuzhen Xiao,* Dongsen Mao and Guanzhong Lu*
2 3
The effects of the preparation method on the properties of Cu/ZnO/Al O catalysts for L-phenylalanine
methyl ester hydrogenation to L-phenylalaninol were investigated in detail, including the precipitation
method and conditions (the aging time, calcination temperature and so on), with the help of ICP-OES,
2 2 2
N and N O adsorption, XRD, H -TPR and TEM techniques. The results show that physicochemical
properties of the catalysts are greatly affected by the preparation method and conditions. The uniform
size distribution of CuO species can be obtained by fractional co-precipitation. The appropriate aging
time is 2 h, and the catalyst aged for 2 h has the largest metallic copper surface area (SCu) and surface
copper amount and the smallest CuO crystallites. The lower calcination temperature is favorable for
2 4
increasing the surface area and metallic copper surface area of the catalyst. The spinel structure CuAl O
phase can form after calcination at 550 °C. The turnover frequency (TOF) values of L-phenylalaninol
formed using different catalysts indicate the structurally sensitive character of the title reaction, and SCu
Received 17th November 2013,
Accepted 21st January 2014
is not the sole cause affecting the catalytic activities of the catalysts. B-TOF on the basis of the active
0
sites (Cu ) in the boundary between CuO and ZnO or Al
2
O
3
was proposed; the relationships of B-TOF
catalyst prepared by
2 3
with dCuO (particle size of CuO) and SCu were established. Using the Cu/ZnO/Al O
DOI: 10.1039/c3cy00937h
fractional co-precipitation with aging at 70 °C for 2 h and calcination at 450 °C for 4 h, 83.6% selectivity
www.rsc.org/catalysis
to L-phenylalaninol without racemization was achieved.
In recent years, increasing attention has been given to
catalytic hydrogenation that is another attractive approach
1
. Introduction
Chiral amino alcohols, especially L-phenylalaninol, are versatile
intermediates and can be widely applied in pharmaceutical
chemistry, fine chemistry and resolution of racemic mixtures,
for obtaining chiral amino alcohols. Noble metal catalysts
under relatively mild reaction conditions were used, such as
19–21
3,22
Ru oxide
or its complexes.
However, the catalytic systems
1–3
etc.
They are also popularly used as efficient catalysts for
mentioned above are also not commercially available.
Undoubtedly, it is necessary to design a highly efficient and
stable catalyst for improving the catalytic hydrogenation of
chiral amino esters in order to avoid these drawbacks.
It is well known that the copper-based catalysts exhibit an
excellent catalytic performance for hydrogenation of esters
4–9
asymmetric synthesis reactions. Therefore, the formation of
chiral amino alcohols has become the subject of considerable
interest. Generally, chiral amino alcohols can be prepared by
the catalytic aminolysis of epoxides with excess amines at
elevated temperatures
acids or esters.
10–14
and the reduction of the corresponding
15–18
23–28
In the latter process, the metal hydrides are
under relatively harsh conditions,
but hydrogenation of
29
used as reducing agents, which are reactive at low temperatures
thereby retaining the integrity of the stereogenic center. However,
most of these metal hydrides are only utilized in the laboratory to
produce chiral amino alcohols on a 100–150 g scale.
chiral esters has been barely reported. Brands et al. reported
that copper-containing catalysts can catalyze the selective
hydrogenation of carbon–oxygen bonds and are relatively
inactive for carbon–carbon bond hydrogenolysis. We have
2 3
used Cu/ZnO/Al O as the catalyst for hydrogenation of
a
Research Institute of Applied Catalysis, School of Chemical and Environmental
L-phenylalanine methyl ester to L-phenylalaninol, and a 69.2%
yield of L-phenylalaninol with an ee value of 99.84% was
Engineering, Shanghai Institute of Technology, Shanghai 20023, PR China.
E-mail: gzhlu@ecust.edu.cn, jerryxiaozh@163.com; Fax: +86 21 60879111
3
0
b
achieved. However, the selectivity to L-phenylalaninol is still
lower. Previous research studies showed that the catalytic
properties of Cu/ZnO/Al O catalysts are affected by the
Key Laboratory for Advanced Materials and Research Institute of Industrial
Catalysis, East China University of Science and Technology, Shanghai 200237,
PR China
2
3
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2
+
2+
3+
preparation and reduction activation method of mixed
(c) The mixture solution of Cu , Zn and Al and the
3
1
32
+
oxides. For instance, Li and Inui found that the pH value
of the reaction medium altered the phase composition of the
catalyst, and the precipitation temperature affects the precipi-
tation kinetics of the precursors of catalysts, while aging tem-
0.5 M Na solution were added to the precipitating reactor
under stirring at 70 °C, while the pH value of the reaction
medium was kept at 7.5 from beginning to end.
(d) The mixture solution of Cu and Zn and the 0.5 M Na
solution were co-precipitated at 70 °C and pH 7.5 under
2
+
2+
+
3
3
perature determined phase inter-dispersion. Figueiredo et al.
reported that the inter-dispersion of oxide phases was
3+
+
stirring. And the Al solution and 0.5 M Na solution were
co-precipitated under the same precipitation conditions
described above. Then two solutions obtained above were
mixed under the same conditions.
Then, these reaction media were aged under stirring for 5 h
at 70 °C and cooled statically for 1 h. After filtration and
washing with de-ionized water until the filtrate was neutral,
the precursor was dried at room temperature for 12 h, then
dried at 120 °C for 24 h, heated to 450 °C at 5 °C min and
calcined at 450 °C for 4 h. The calcined catalyst was pressed
and crushed to small-sized particles of 0.45–0.85 mm (20–40 mesh).
The prepared catalysts were labeled as CZA-x-y-t: x repre-
sents the precipitation method (a, b, c and d), y represents
the aging time (5 h) at 70 °C, and t represents the calcination
temperature (450 °C) for 4 h. Hence, four catalysts above are
CZA-(a, b, c, d)-5-450.
3
4
influenced by the precipitation methods. Jung et al.
suggested that precursor structures would be changed during
the aging process, and the metallic copper surface area was
3
5
decreased due to a long aging time. Fujita et al. investigated
the effects of calcination and reduction conditions on the cata-
lytic performance of the Cu/ZnO catalyst for the methanol syn-
thesis from CO and found that a highly efficient catalyst could
2
−
1
be prepared from aurichalcite by controlling the preparation con-
ditions. The reported results mentioned above show that the cata-
lytic properties of Cu/ZnO/Al O are affected remarkably by the
2
3
preparation method and conditions.
Herein, the Cu/ZnO/Al O catalysts were prepared using
2
3
different precipitation methods and conditions, their physi-
cochemical and catalytic properties for hydrogenation of
L-phenylalanine methyl ester to L-phenylalaninol were investi-
gated, and the overall reactions are given in Scheme 1. The
effects of the physicochemical properties of catalysts on their
catalytic performances were discussed.
For the CZA-d-y-450 catalyst, the aging time (y) was
changed from 0 to 5 h. The CZA-d-2-t catalyst aged for 2 h at
70 °C was calcined at 350–750 °C for 4 h.
2.2. Catalyst characterization
2
. Experimental section
The chemical composition of the sample was determined by
inductively coupled plasma-optical emission spectroscopy
2
.1. Catalyst preparation
The Cu/ZnO/Al catalysts with optimal mole ratio of Cu: Zn:
Al = 1 : 0.3 : 1 were prepared with Cu(NO ) ·3H O, Zn(NO ) ·6H O,
(
ICP-OES; Perkin Elmer, Optima 7000 DV). The powder X-ray
2 3
O
diffraction (XRD) patterns of the catalysts were recorded
using a PANalytical X'Pert Pro MRD X-ray diffractometer
3
2
2
3 2
2
Al(NO ) ·9H O and Na CO (A.R., Sinopharm Chemical Reagent
Ltd.), and the preparation procedures were as follows:
.0 M Cu(NO ) , 1.0 M Zn(NO ) , 1.0 M Al(NO ) and
3
3
2
2
3
(
The Netherlands) with CuKα radiation (λ = 0.154056 nm)
3
0
operated at 40 kV and 40 mA. The average crystalline sizes of
catalysts (dCuO) were calculated by the Scherrer equation
based on the diffraction peak broadening. The surface areas
1
3
2
3 2
3 3
0
.5 M Na CO aqueous solutions were prepared first. Four
2 3
different preparation methods were used as follows:
of the catalysts were measured by N adsorption at −196 °C
2
+
(
a) 0.5 M Na aqueous solution was added to a mixture
using a Micrometrics ASAP 2020 apparatus and calculated by
the Brunauer–Emmett–Teller (BET) method.
2
+
2+
3+
solution containing Cu , Zn and Al under stirring at
0 °C until the pH value of the reaction medium reached 7.5.
7
The metallic copper surface area (SCu) of the catalyst was
determined using a nitrous oxide chemisorption method
2
+
2+
3+
(
b) The mixture solution of Cu , Zn and Al was added
+
to the 0.5 M Na solution under stirring at 70 °C until the pH
value of the reaction medium reached 7.5.
33
called the reactive frontal chromatography (RFC) technique.
For example, 200 mg of sample was reduced in a 5% H /He
2
−
1
stream at a heating rate of 5 °C min to 300 °C and
maintained for 1 h. After that, the reactor was purged with
the pure He stream and cooled down to 60 °C, and then N O
2
titration was carried out. The surface copper atoms density of
1
9
2
1
2
.46 × 10 copper atoms per m assuming Cu : N O = 2 : 1
was used for the calculation of the copper surface area. The
copper dispersion (DCu) was defined as the amount of the
exposed copper in relation to the total amount of copper
atoms of the catalyst.
Transmission electron microscopy (TEM) images were
obtained using a JEOL 1400 microscope operated at 100 kV.
The samples were suspended in ethanol and supported onto
a holey carbon film on a Cu grid.
Scheme
1 (a) Hydrogenation of L-phenylalanine methyl ester to
L-phenylalaninol and (b) the side reaction of L-phenylalanine methyl
ester self-polymerization.
36
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H -temperature-programmed reduction (H -TPR) was
performed using a continuous-flow apparatus equipped with
a thermal conductivity detector (TCD). 30 mg of catalyst was
by HPLC with a chiral column, which indicates that the ee value
is ~100% and chirality of the reactant can be well maintained
after the reaction.
2
2
used and pretreated at 350 °C for 1 h under a N flow of
2
−
1
3
0 mL min . After it was cooled to 50 °C under N , it was
2.5. Boundary-turnover frequency (B-TOF)
2
−1
2 2 2
flushed by a 10% H /N flow of 30 mL min instead of pure N ,
and TPR of the sample was run from 50 to 550 °C at 5 °C min .
Herein, we propose the boundary-turnover frequency (B-TOF)
of L-phenylalaninol formation, which means the number of
L-phenylalaninol molecules formed per second per metallic
copper atom in the boundary between CuO and ZnO or
Al O . Being different from TOF, B-TOF is calculated on the
Cu sites in the boundary rather than on all Cu sites on the
whole surface. The Cu sites in the boundary can be described
ideally as the following figure:
−1
2
.3. Catalytic activity testing
The activity of the catalyst for L-phenylalanine methyl ester
hydrogenation was tested in a 500 mL stainless steel auto-
clave under stirring at a speed of 500 rpm. After 1.0 g of
catalyst (20–40 mesh) was packed in the reactor, this reactor
2
3
was flushed with 4 MPa H
catalyst was reduced in 1 MPa H
2
to expel air 4 times and the
at 250 °C for 4 h, and
2
then the reactor was cooled to room temperature. 1.5 g of
L-phenylalanine methyl ester (liberated from the L-phenylalanine
methyl ester hydrochloride with 0.5 M Na CO aqueous solution
2 3
and extracted by ethyl acetate) diluted in 150 mL ethanol was
introduced, that is, L-p/Cat. = 1.5 (mass ratio). The typical reac-
2
tion conditions were 4 MPa of H and 110 °C. After the reaction
was over, the reactor was cooled to room temperature, and
then the pressure was released. The catalyst was separated by
centrifugation, and the products were analyzed by HPLC and
NMR (Bruker, AVANCE III 500 MHz).
HPLC (high performance liquid chromatography) analysis
was done using an Agilent 1260 Infinity equipped with an
ultraviolet detector and a column (Poroshell 120 EC-C18,
0
In this model, CuO particles (or Cu particles) are located
0
on the surface of Zn0.3AlOx oxide. The Cu particles are
5
0 × 4.6 mm, 2.7 μm particle size). The operating conditions
hypothesized as round particles, and their particle diameter
was determined by the XRD patterns and the Scherrer equation.
−
1
of HPLC were as follows: the mobile phase was 0.05 mol L
ammonium acetate aqueous solution (pH = 5.0) containing
5
0
The active sites are Cu atoms in the boundary between CuO
−
1
0
vol.% methanol, the flow rate was 0.6 mL min , the detec-
and the support, namely, the Cu atoms on the circumference
0
0
tion wavelength was 254 nm and the column temperature
was 35 °C. Experimental errors for the conversion and selec-
tivity are within ±2%. The conversion of L-phenylalanine
methyl ester (L-p) (C), yield of L-phenylalaninol (L-p-ol) (Y)
and chemoselectivity (S) were calculated as follows:
C (%) = (the mass of L-phenylalanine methyl ester converted/
total mass of L-phenylalanine methyl ester in the feed) × 100%
Y (%) = (the mass of L-phenylalaninol formed actually/the
mass of L-phenylalaninol formed theoretically) × 100%
S (%) = (Y/X) × 100%
of the round Cu particles on Zn Al O . The Cu atoms in the
0.3 1 x
boundary between CuO and Zn Al O were calculated by:
0
.3
1 x
2
N = SCuO/π(d/2) × (πd/2r) = 2 × SCuO/(d × r)
SCuO = the surface area of CuO on the surface;
d = CuO average particle diameter determined by XRD data;
r = the radius of the CuO molecule, which is approxi-
2
−
mately equal to the O ion radius (0.138 nm).
SCuO = (SCu/a ) × a = S × a /a
1
1
2
Cu
2
SCu = the surface area of copper on the surface;
a1 = the surface area of the copper atom (7.11 × 10 m );
a2 = the surface area of the CuO molecule, which
−
20
2
The ee value of the product was determined by HPLC with
a chiral column (CHIRALPAK ID-3, 150 × 4.6 mm, 5 μm particle
size) under the operating conditions: the mobile phase was the
2
−
is approximately equal to the surface area of the O ion
−
20
2
(5.98 × 10 m ).
−1
mixture solution (water/methanol = 70: 30 (v/v), 0.6 mL min )
containing 3% triethylamine, the detection wavelength was 3. Results and discussion
2
58 nm and the column temperature was 40 °C.
3.1. Effect of precipitation methods
To ascertain an appropriate reaction time, the effect of the
reaction time on the title reaction over the CZA-d-5-450
catalyst was tested and the results are shown in Fig. 1A. The
results show that the selectivity to L-phenylalaninol (L-p-ol) is
hardly changed (~67.9%) and the conversion of L-phenylalanine
methyl ester (L-p) is increased gradually with increasing
reaction time. It reaches 96.3% after 4 h of reaction and ~100%
2
.4. The structure and ee value of the product
1
The H NMR spectrum of product L-phenylalaninol was
obtained using a Bruker AVANCE-III 500. HNMR (CDCl3):
1
7
.21–7.35 (5H, m, Ph–H), 3.42–3.68 (2H, m, –CH
2
–O–), 3.15
(1H, s, –CH–N–), 2.53–2.84 (2H, m, CH –Ph), 1.83 (2H, b, –NH ).
2
2
In the product solution, only L-phenylalaninol could be detected
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for 5 h. The results demonstrate that the L-phenylalaninol
formed undergoes no further reaction at the applied condi-
tions, that is to say, the surface Cu atoms are inactive for deep
hydrogenation of L-phenylalaninol. Hence, the selectivity to
L-phenylalaninol is not changed with the reaction time.
The results in Fig. 1B show that the precipitation method
affects obviously the performance of the CZA catalyst, and
among the four catalysts, the CZA-d-5-450 catalyst exhibits
the highest activity, for instance, after reaction for 10 min,
67.9% selectivity and 25% conversion can be obtained.
0
The catalytic performances of the CuZn0.3AlO
x
(CZA)
The effect of the precipitation method on some physico-
chemical properties of the CZA catalysts is shown in Table 1.
The elemental analysis results indicate that the precipitation
method hardly affects the compositions of those catalysts.
However, the BET surface area, average size of CuO crystallites
(dCuO), metallic copper surface area (SCu) and copper disper-
sion (DCu) of the CZA catalyst are obviously affected by the
precipitation method. The BET surface areas are varied in
the order of CZA-d-5-450 (159 m g ) > CZA-c-5-450 (115) >
CZA-b-5-450 (82.2) > CZA-a-5-450 (47.6), resulting in the
similar variation of SCu and DCu values of these catalysts. For
instance, for dCuO estimated by the Scherrer equation on the
basis of the XRD diffraction peak of CuO, CZA-a-5-450 is
catalysts prepared by different precipitation methods for
L-phenylalanine methyl ester hydrogenation to L-phenylalaninol
are presented in Fig. 1B. It can be seen that 100% conversion
of L-phenylalanine methyl ester (L-p) was obtained at 110 °C
and 4 MPa of H2 for 5 h, and when the reaction time was
decreased to 10 min, only 21.3–25.0% conversion was obtained.
However, the reaction time hardly affected the selectivity to
L-phenylalaninol (L-p-ol), which is only 50.6–67.9%.
2
−1
22.3 nm and CZA-d-5-450 is only 10.5 nm. The results above
show that the CZA sample (as CZA-d-5-450) prepared by
mixing the co-precipitate of Cu and Zn with the Al-precipitate
possesses larger dispersion and surface area of metallic
33
copper, which are similar to the results reported by Figueiredo.
Different precipitation methods mean that the Cu, Zn and
Al ions are deposited at different pH environments. Based on
2
the different solubility product constants (Ksp) of Cu(OH) ,
2
+
Zn(OH)
(
2 3
and Al(OH) , the beginning precipitation pH of Cu
3
+
2+
as well as Al ) is near 4 and that of Zn is 6. During the
precipitation of CZA-a-5-450, pH of the reaction medium was
increased from 2.5 to 7.5, which would produce an inhomo-
2
+
2+
geneous precipitation of Cu with Zn ions, resulting in
weaker interaction of CuO and ZnO species. Hence, larger
sizes and lower dispersion of CuO species were obtained for
the CZA-a-5-450 sample.
During the precipitation of CZA-b-5-450, pH of the reaction
medium was decreased from 11.5 to 7.5 and homogeneous
distribution of the CuO species was obtained, but the size of
the CuO phase is larger than that in CZA-c-5-450 (deposited at
unvaried pH 7.5) and CZA-d-5-450, because each component
can be completely precipitated under the higher pH value
solution. In the preparation process of CZA-d-5-450, the
co-precipitate of Cu/Zn and the Al-precipitate were formed
separately, and then two precipitates were mixed uniformly,
Fig. 1 The effect of reaction time on the L-phenylalanine methyl ester
hydrogenation over the CZA-d-5-450 catalyst (A); effect of the
precipitation method on the performance of the CZA catalyst (B) at
1
2
10 °C and 4 MPa of H for 10 min or 5 h (L-p/Cat. = 1.5, wt.).
2 3
Table 1 Physicochemical properties of the Cu/ZnO/Al O catalysts prepared by different precipitation methods
a
Molar composition
b
3c
d
S
BET
d
(nm)
CuO
S
Cu
D
(%)
Cu
TOF × 10
B-TOF
2
−1
2
−1
−1
−1
Catalyst
Cu
Zn
Al
(m g
)
(m g
)
(s
)
(s
)
CZA-a-5-450
CZA-b-5-450
CZA-c-5-450
CZA-d-5-450
1.03
1.06
1.05
1.00
0.30
0.30
0.28
0.28
0.97
0.94
0.97
1.02
47.6
82.2
115
159
22.3
13.9
11.6
10.5
4.6
6.3
8.1
2.2
3.0
3.9
5.5
13.5
11.2
10.3
8.4
0.492
0.254
0.197
0.140
11.5
a
b
c
Determined by the ICP-OES method.
DCu = exposed Cu atoms/total Cu atoms. Turnover frequency (TOF) represents the number of
d
L-phenylalaninol molecules formed (reaction for 10 min) per second per surface metallic copper atom. B-TOF represents the number of
L-phenylalaninol molecules formed (reaction for 10 min) per second per Cu atom in the boundary between CuO and ZnO or Al O .
2 3
0
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which is not only beneficial for the dispersion between
Cu–Zn–O and Al oxide species but also enhances the interac-
tion of CuO with ZnO, resulting in the higher dispersion and
smaller size of CuO crystallites in the CZA-d-5-450 sample.
Therefore, it is easily understood that the CZA-d-5-450 sample
exhibits higher catalytic activity and selectivity to the product,
because a higher dispersion of Cu usually results in a higher
sample, the homogeneous and smallest particle size can
be observed.
2
The H -TPR profiles of calcined catalysts are shown in
Fig. 4, and four samples exhibit four different kinds of H2
reduction (consumption) peaks at 150–275 °C, which can be
deconvoluted to two (α and β) peaks and their peak positions
and contributions are summarized in Table 2. Since ZnO and
3
7–39
40
catalytic performance for Cu-based catalysts.
Al O cannot be reduced in this temperature region, these
2
3
Fig. 2 shows the XRD patterns of the catalysts prepared
by different precipitation methods. The results show that no
diffraction peaks of Al O and ZnO can be detected,
reduction peaks are only ascribed to the reduction of CuO
species. The low temperature peak (α peak) is ascribed to the
reduction of highly dispersed CuO, and the peak at higher
2
3
38,41
suggesting that Al
2
O
3
and ZnO phases are amorphous or
temperature (β peak) is due to the reduction of bulk CuO.
Compared with the reduction (reduction temperature is usually
highly dispersed. The diffraction peaks of CuO can be observed
at 2θ = 35.6°, 38.8° (JCPDS 80-1268) in these catalysts. From
CZA-a-5-450 to CZA-d-5-450, the diffraction peaks of CuO
become weaker and broader, indicating a decrease in the CuO
crystal size (Table 1).
4
2
at ~300 °C) of pure CuO, CuO species in the CZA catalysts
can be reduced at much lower temperatures, which suggests
2 3
Fig. 3 shows the TEM images of the Cu/ZnO/Al O catalysts
prepared by different precipitation methods. It can be seen
that the precipitation methods have a great influence on the
particle size and the distribution of catalysts. Among the four
samples, the CZA-b-5-450 particle size seems generally rela-
tively larger because of the higher pH value of the reaction
medium, and the CZA-c-5-450 and CZA-d-5-450 samples
exhibit smaller sizes. However, CZA-c-5-450 exhibits uniform
size distribution with some bigger particles. For the CZA-d-5-450
Fig.
2 2 3
4 H -TPR profiles of Cu/ZnO/Al O catalysts prepared by
different precipitation methods.
Table 2 Top temperatures and relative peak areas in the TPR patterns
of Cu/ZnO/Al
2
O
3
catalysts
a
Catalyst
T
α
(°C)
T
β
(°C)
A /(A
α α
+ A
β
)
(%)
CZA-a-5-450
CZA-b-5-450
CZA-c-5-450
CZA-d-5-450
206
191
189
206
234
218
212
35.5
43.5
69.4
—
Fig.
2 3
2 XRD patterns of the Cu/ZnO/Al O catalysts prepared by
a
different precipitation methods.
A is the reduction peak area.
Fig. 3 TEM images of (A) CZA-a-5-450, (B) CZA-b-5-450, (C) CZA-c-5-450 and (D) CZA-d-5-450.
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that the presence of ZnO can enhance the reducibility of CuO
species.
shown in Fig. 5. It can be seen that 100% conversion of
L-phenylalanine methyl ester can be obtained at 110 °C and
4
1,43–46
The results in Fig. 4 show that the precipitation method
affects remarkably the reduction properties of CZA catalysts.
For CZA-a-5-450, CZA-b-5-450 and CZA-c-5-450 catalysts, there
are obviously two reduction peaks, and in the TPR curve of
the CZA-d-5-450 catalyst, two reduction peaks (α, β) have
overlapped, which indicate that the former three samples
contain two kinds of CuO species and the latter exists as
almost one kind of CuO species. Although the top tempera-
ture of the β peak is related to the X-ray diffraction peak
intensity of CuO (Fig. 2), we find that the higher the β peak
temperature is, the stronger the diffraction peak of CuO is.
That is to say, the β peak is attributed to the reduction of
bulk CuO. Likewise, the larger the α peak area is, the larger
the surface area of Cu (SCu, Table 1) is. In the TPR curve of
the CZA-d-5-450 catalyst, only one peak is observed and the α
peak is completely overlapping with the β peak, suggesting
the presence of uniform particles of CuO species. Therefore
CZA-d-5-450 possesses the largest SCu and the smallest
crystallite size of CuO among the four samples.
2
4 MPa of H for 5 h, and over the CZA-d-2-450 aged for 2 h
the selectivity to L-phenylalaninol reaches the highest
(83.1%). When the reaction time was shortened to 10 min,
the product selectivity curve is similar to that of 5 h. However,
the conversions are decreased to 22–25% due to shorter reac-
tion time. With the increase in the aging time from 0 h to 2 h,
the conversion is increased from 22.1 to 25.4%, and with a
further increase in the aging time, the conversion is no longer
obviously changed. Therefore, the appropriate aging time is
2 h for the CZA-d-y-450 catalyst.
The main physicochemical properties of the CZA-d-y-450
catalysts with different aging times are shown in Table 3. The
results show that the aging time hardly affects the Cu, Zn
and Al contents in the catalyst, but its BET surface area is
2
−1
influenced remarkably and increased from 41.2 to 159 m g
with the increase in aging time from 0 to 5 h. The average
crystallite size (dCuO) of CuO declines from 23.0 to 10.2 nm
with an increase in the aging time from 0 to 2 h, and with a
further increase in the aging time, dCuO is no longer signifi-
Relating the catalytic activity data in Fig. 1 with SCu values
in Table 1, it can be seen that the selectivity to L-phenylalaninol
increases with the increase in the SCu values, but there is no
linear relationship between selectivity and SCu. This indicates
cantly changed. The metallic copper surface area (S ) of
Cu
2
−1
CZA-d-2-450 aged for 2 h reaches its maximum (16.6 m g ),
and the corresponding DCu (exposed copper atoms/total copper
atoms) is also at its maximum (7.9%). In addition, we still
2 3
that the activity of the Cu/ZnO/Al O catalyst is not only related
to SCu but also to other factors, such as the promotional role of
ZnO on the Cu sites, which can enhance the reducibility of the
copper oxide phase with the help of the synergetic effect or the
3
0,40
interaction between metallic copper and zinc oxide.
It is
well known that the interaction between the components of a
catalyst can be selectively affected by different precipitation
3
3
methods. For the CZA-d-5-450 catalyst prepared by mixing
the co-precipitate of Cu and Zn with the Al-precipitate, the
interaction of CuO (or Cu) with ZnO ought to be stronger than
other catalysts, which brings well into play the promotional
role of ZnO.
3
.2. Effect of precipitate aging time
Fig. 5 Effect of the aging time at 70 °C on the performance of the
CZA-d-y-450 catalyst for the hydrogenation of L-phenylalanine methyl
The CZA-d-y-450 sample was used as the model catalyst, and
the effect of the aging time on its catalytic performance is
2
ester at 110 °C and 4 MPa of H for 10 min and 5 h (L-p/Cat. = 1.5, wt.).
Table 3 Physicochemical properties of the CZA-d-y-450 catalysts prepared with different aging times
a
Molar composition
b
3c
d
Aging time
(h)
S
BET
d
(nm)
CuO
S
Cu
D
(%)
Cu
TOF × 10
B-TOF
2
−1
2
−1
−1
−1
Catalyst
Cu
Zn
Al
(m g
)
(m g
)
(s
)
(s
)
CZA-d-0-450
CZA-d-1-450
CZA-d-2-450
CZA-d-3-450
CZA-d-4-450
CZA-d-5-450
0
1
2
3
4
5
0.97
1.02
1.03
1.02
1.07
1.04
0.31
0.30
0.29
0.29
0.30
0.28
1.02
1.07
0.98
1.01
1.01
1.02
41.2
53.4
92.0
99.7
126
159
23.0
12.6
10.2
10.7
10.4
10.5
7.4
12.4
16.6
14.9
13.5
11.5
3.5
5.9
7.9
7.1
6.5
5.5
10.9
8.0
7.0
7.5
7.8
8.4
0.402
0.160
0.114
0.140
0.121
0.140
a
b
c
Determined by the ICP-OES method.
DCu = exposed Cu atoms/total Cu atoms. Turnover frequency (TOF) represents the number of
d
L-phenylalaninol molecules formed (reaction for 10 min) per second per surface metallic copper atom. B-TOF represents the number
of L-phenylalaninol molecules formed (reaction for 10 min) per second per Cu atom in the boundary between CuO and ZnO or Al O .
2 3
0
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found that the color of the precursor varied with the aging
time, which means that the structure of the precursors changes
34,47
during the aging process.
in Fig. 5 with SCu values in Table 3, it can be seen that the
selectivity to L-phenylalaninol increases with an increase in SCu
Relating the catalytic activity data
,
and the CZA-d-2-450 catalyst with the largest SCu shows the
highest catalytic performance.
The XRD patterns in Fig. 6 show that no diffraction peaks
2 3 2 2 4
of Al O , ZnO and Cu Al O phases can be observed in the
CZA-d-y-450 samples, like the XRD curve of the CZA-d-5-450
catalyst (Fig. 2). And the diffraction peaks of the CuO phase
markedly decreased with an increase in the aging time from
0
to 2 h, due to a change of dCuO. With a further increase in
the aging time (>2 h), the dCuO values slightly increase with
the aging time, but its variation is unapparent. Furthermore,
the TEM images in Fig. 7 show that the particle size of
CZA-d-2-450 is smaller than that of CZA-d-5-450, suggesting
that increasing aging time (>2 h) slightly contributes to the
growth of catalyst particles.
Fig. 7 TEM images of (A) CZA-d-2-450 and (B) CZA-d-5-450.
2
Fig. 8 shows the H -TPR profiles of the catalysts prepared
with different aging times. All of the samples exhibit a broad
reduction peak at 150–275 °C. The CZA-d-0-450 sample
exhibits the highest reduction temperature, and the top
reduction temperature of the CZA-d-2-450 sample aged for
2
h is the lowest. For other samples aged for >2 h, the top
temperature of reduction peak rises with an increase in the
aging time, although their XRD patterns are hardly changed
4
8
(
Fig. 6). Kniep et al. reported that continuous precipitation
aging would lead to a decreasing amount of Zn in the copper
clusters for the Cu/ZnO catalysts. This shows that a long
aging time is not beneficial for reducibility of copper oxide,
because decreasing the Zn amount in CuO crystallites
weakens the synergetic effect and hydrogen spillover between
metallic copper and zinc oxide.
Fig. 8
2
H -TPR patterns of the CZA-d-y-450 catalysts prepared with
3
.3. Effect of calcination temperature
different aging times (y).
On the basis of the results above, the CZA-d-2-450 prepared
with aging time of 2 h was used a model catalyst to study the
effect of the calcination temperature on its catalytic
performance, and the results are shown in Fig. 9. The results
show that 100% conversion of L-phenylalanine methyl ester
can be obtained for all catalysts after 5 h of reaction, and the
highest selectivity to L-phenylalaninol can be obtained over
the CZA-d-2-450 catalyst calcined at 450 °C. If the calcination
temperature was >450 °C, the catalytic activity of CZA-d-2-t
would drastically decrease with an increase in calcination
temperature. When the reaction time was shortened to
1
5
1
0 min, the product selectivity curve is similar to that after
h of reaction, and the conversions are decreased to
7–25.3%. Among those catalysts, the CZA-d-2-450 catalyst
calcined at 450 °C exhibits the highest reactant conversion
and product selectivity. Therefore, the appropriate calcination
temperature is 450 °C.
For the CZA-d-2-450 catalyst, the effect of the reaction time
on the conversion and product selectivity is given in Fig. 10.
The results show that the selectivity to L-phenylalaninol is
Fig. 6 XRD patterns of the CZA-d-y-450 catalysts prepared with
different aging times (y).
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contrary, the average size of CuO crystallites (dCuO) is increased
from 9.7 nm (CZA-d-2-350) to 27.3 nm (CZA-d-2-750). The
growth of crystal grain (and/or the agglomeration of particles)
and the formation of spinel oxide are closely related to the
calcination temperature, resulting in the variation of SBET, SCu
,
4
4
DCu and dCuO.
Comparing with CZA-d-2-350, CZA-d-2-450 with lower
surface area and SCu exhibits the higher catalytic performance,
indicating that SCu is not the sole decisive factor for the
47
catalytic performance of Cu/ZnO/Al O . Spencer et al. proposed
2
3
that hydrogen dissociation on zinc oxide, followed by hydrogen
spillover to copper, is significant for the Cu/ZnO/Al catalysts.
found that the strength and nature of the interac-
2 3
O
4
9,50
Sun et al.
Fig. 9 Effect of the calcination temperature (t) on the performance of
the CZA-d-2-t catalyst for the hydrogenation of L-phenylalanine
tion between dispersed copper and other oxide species rather
than only their surface area are decisive for the catalytic
methyl ester at 110 °C and 4 MPa of H
.5, wt.).
2
for 10 min and 5 h (L-p/Cat. =
1
51
activity, such as the reported Cu/ZrO2 catalyst. Wu et al.
suggested that the encapsulation effect becomes apparent with
an increase in the calcination temperature, which can reinforce
the interaction between metallic copper and zirconia in the
hardly changed (~83.6%) and the conversion of L-phenylala-
nine methyl ester is gradually increased with the reaction
time. The conversion reaches 98.3% for 2 h and 100% for
2
Cu/ZrO catalysts. And undesired aggregations would occur
3
h.
The physicochemical data in Table 4 show that the BET
under relatively high calcination temperatures. Thus, we specu-
late that the interaction between CuO and ZnO plays an
important role in the catalytic activity, and the role of ZnO in
2
−1
surface area of CZA-d-2-t is decreased from 125 to 39.9 m g
with an increase in calcination temperature from 350 to
50 °C, while the variation trend of SCu and DCu is similar to
2 3
the Cu/ZnO/Al O catalyst cannot be ignored. It is reasonable
7
that the CZA-d-2-350 sample with the largest SCu is not related
to the highest catalytic performance, because of the weaker
interaction between metallic copper and ZnO when the calcina-
tion temperature is too low. In addition, the Tammann temper-
the xBET surface area with the calcination temperature. On the
52
ature of CuO is only 558 °C; too high calcination temperature
might result in the sintering of CuO crystallites. Therefore,
the calcination temperature of 450 °C is appropriate for the
2 3
Cu/ZnO/Al O catalyst.
The XRD patterns of the catalysts calcined at different
temperatures are shown in Fig. 11. It can be seen that no
2 3
diffraction peaks of the Al O and ZnO phases can be
observed for all samples, which is similar to the results in
Fig. 2 and 6 (the effects of precipitation methods and aging
time). Weaker and broader diffraction peaks of CuO (JCPDS
80-1268) appear at 2θ = 35.5° and 38.8° in the XRD curve of
CZA-d-2-350. With an increase in the calcination temperature,
the diffraction peaks of the CuO phase become stronger and
sharper, indicating an increase in the crystallization degree
of CuO. After this catalyst was calcined at 550 °C, the
Fig. 10 The effect of reaction time on the L-phenylalanine methyl
ester hydrogenation over the CZA-d-2-450 catalyst at 110 °C and
4
2
MPa of H (L-p/Cat. = 1.5, wt.).
2 3
Table 4 Physicochemical properties of the Cu/ZnO/Al O catalysts calcined at different temperatures
a
Molar composition
b
3c
d
Calcination
temp. (°C)
S
BET
d
(nm)
CuO
S
Cu
D
(%)
Cu
TOF × 10
S
Cu·TOF
B-TOF
2
−1
2
−1
−1
2
−1 −1
−1
Catalyst
Cu
Zn
Al
(m g
)
(m g
)
(s
)
(m s
g
)
(s
)
CZA-d-2-350
CZA-d-2-450
CZA-d-2-550
CZA-d-2-650
CZA-d-2-750
350
450
550
650
750
1.02
1.03
1.01
1.03
1.00
0.31
0.29
0.31
0.32
0.30
1.01
0.98
1.00
1.06
1.01
125
9.7
10.2
18.3
23.9
27.3
24.5
16.9
12.9
6.8
11.7
8.1
6.2
3.3
2.3
3.5
7.0
7.9
10.7
13.3
0.0637
0.120
0.102
0.073
0.064
0.052
0.114
0.246
0.415
0.576
92.0
82.1
47.6
39.9
4.8
a
b
c
Determined by the ICP-OES method.
D
Cu = exposed Cu atoms/total Cu atoms. Turnover frequency (TOF) represents the number of
d
L-phenylalaninol molecules formed (reaction for 10 min) per second per surface metallic copper atom. B-TOF represents the number of
L-phenylalaninol molecules formed (reaction for 10 min) per second per Cu atom in the boundary between CuO and ZnO or Al O .
2 3
0
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Fig. 13
different temperatures (t).
H
2
-TPR profiles of the CZA-d-2-t catalysts calcined at
Fig. 11 XRD patterns of the CZA-d-2-t catalysts calcined at different
temperatures (t).
Table 5 The top temperatures of reduction peaks and their contributions
CuAl O spinel phase was formed, and its diffraction peaks
2
4
to TPR patterns of Cu/ZnO/Al
O
2 3
calcined at different temperatures
(
JCPDS 33-0448) are located at 2θ = 31.2°, 36.8°, 44.8° and
5.7° (Fig. 11). The higher the calcination temperature is, the
higher the crystallization degree of CuAl produced. After
5
a
Catalyst
T
α
(°C)
T
β
(°C)
A /(A
α α
+ A
β
)
(%)
2 4
O
CZA-d-2-750
CZA-d-2-650
CZA-d-2-550
CZA-d-2-450
CZA-d-2-350
209
207
201
196
191
228
232
224
—
79.0
87.9
93.3
~100
100
calcination at 750 °C, the CZA-d-2-750 sample was agglomerated
significantly (Fig. 12).
2
The H -TPR profiles of the CZA-d-2-t calcined at different
—
temperatures are shown in Fig. 13. It can be seen that all
of the Cu/ZnO/Al O catalysts exhibit a broad peak of H con-
a
2
3
2
Aα and A represent the area of α and β peaks, respectively.
β
sumption at 150–275 °C, which can be deconvoluted into two
peaks (α and β peaks). The positions and the contributions
of these reduction peaks are summarized in Table 5. Since
4
5
CuO species. The results show that decreasing the calcina-
tion temperature causes the positions of the reduction peaks
to shift toward lower temperatures accompanied by an
increase in the fraction of the α peak, which indicates that
the amount of easily reducible well-dispersed copper oxide
increases with a decline in the calcination temperature. For
the sample calcined at 350 °C, its reduction peak reaches the
lowest temperature and its peak area is also the smallest,
which should be ascribed to a large amount of nano-CuO
species encapsulated in bulk of the catalyst due to a higher
dispersion degree and smaller crystallites of CuO (Table 4).
ZnO, Al
2
O
3
2 3
and CuAl O cannot be reduced at <350 °C, the
peaks of H
2
consumption are ascribed to the reduction of
4. Turnover frequency (TOF) of L-
phenylalaninol formation
To better understand the nature of the activity of the catalyst
active sites, the turnover frequency (TOF) of L-phenylalaninol
formation was used, which means the number of L-phenylalaninol
molecules formed per second per surface metallic copper atom
0
on the basis of the surface area of metallic Cu (SCu). The
results in Fig. 10 show that the reactant conversion is close
to 100% after 2 h of reaction. To appropriately compare the
activity of the active sites of different catalysts in the kinetic
region, the TOFs were calculated on the basis of formed
L-phenylalaninol after 10 min of reaction (that is the data
during the initial 10 min), and the results are listed in
Tables 1, 3 and 4.
Fig. 14A shows the relationship between TOF and the
mean particle size of CuO (dCuO) calculated using the
Fig. 12 TEM images of (A) CZA-d-2-450 and (B) CZA-d-2-750.
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Scherrer equation on the basis of the XRD spectra. The
results show that the TOF value decreases with a decrease in
is because the activity of the Cu-based catalyst is not only
related to the catalytic activity of individual copper atoms
(TOF) but also to the total number of copper atoms on the
catalyst surface, namely, the surface area of Cu (SCu) on the
catalyst. The larger the SCu, the greater the number of active
sites on the catalyst surface. If the SCu·TOF values of catalysts
d
CuO, that is to say, the larger CuO particles possess higher
0
catalytic activities for the title reaction, because the smaller
CuO particles are affected easily by ZnO (or Al O ) near or
2
3
exhibit stronger interaction between CuO and ZnO (or Al
2 3
O ).
This situation has been found in the CuO–ZnO/ZrO catalyst
were compared, we find that S ·TOF of the CZA-d-2-450
2
Cu
4
1,53
2
−1 −1
for CO hydrogenation to produce methanol
and indicates
catalyst is the largest (0.120 m
Cu/ZnO/Al catalysts (Table 4).
It is well known that the catalytic activities of Cu/ZnO/Al O
s
g ) among all of the
2
also the structure-sensitive catalytic hydrogenation character
of the title reaction.
2 3
O
2
3
TOF against the metallic copper surface area (SCu) was
also plotted in Fig. 14B. The results show that TOF of
catalysts are not only dependent on the number of active sites
but also on the Cu dispersion and interaction of Cu/Zn.
However, the active sites of the catalysts are still controversial,
and many different (and conflicting) models have been
L-phenylalaninol formation decreases with an increase in SCu
,
which indicates a decrease in utilization of individual copper
atoms. This result suggests that the activity of the catalyst is
not only related to SCu but also to other causes.
5
4,55
proposed by different researchers.
Many researchers
(including us) thought that the atoms or sites in the boundary
(or interface) between the A component and the B component
(or the active component and support) are mainly catalytic
In our previous works, the L-phenylalaninol selectivity over
the CuO/Al
2 3
O catalyst was only 10.7%, and after adding ZnO
5
6,57
in the CuO/Al O catalyst, the L-phenylalaninol selectivity was
active sites for the supported catalysts,
Cu/ZnO/Al catalyst.
such as the
2
3
3
0
increased obviously. This suggests that ZnO is a very impor-
tant promoter of the CuO/Al catalyst for this reaction.
Therefore, TOF over the Cu/ZnO/Al O catalyst is not only
2 3
O
2
O
3
In general, TOF is calculated on the basis of all surface
metallic atoms or sites, but many surface atoms or sites are
inactive. Hence, the obtained value of TOF often is much
lower. Herein, we propose the turnover frequency on the
basis of the active sites in the boundary, defined as B-TOF,
because the interaction between different oxides (or metal
and support) occurs in the boundary or interface. Once this
2
3
dependent on SCu and dCuO but also on the synergetic effect
or interaction between the metallic copper and the other
oxide components, such as ZnO and Al O .
2
3
Herein, the CZA-d-2-450 catalyst is the most efficient for
−
3
−1
the title reaction, but its TOF is only 7.0 × 10 s , why? This
Fig. 14 The relationship between TOF of L-phenylalaninol formed and
Fig. 15 The relationship between B-TOF of L-phenylalaninol formed
(
A) the mean particle size of CuO (dCuO) and (B) the surface area of
and (A) the mean particle size of CuO (dCuO) and (B) the surface area of
0
0
Cu (SCu) on the Cu/ZnO/Al
2
O
3
catalysts (reaction condition: 110 °C
Cu (SCu) on the Cu/ZnO/Al
2
O
3
catalysts (reaction condition: 110 °C
and 4 MPa of H for 10 min, L-p/Cat. = 1.5, wt.).
2
and 4 MPa of H
2
for 10 min, L-p/Cat. = 1.5, wt.).
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hypothesis can be demonstrated by experimental evidence,
the B-TOF would be widely applied because it can reflect
more realistically the relationship between the catalytic activity
and active sites. Herein, B-TOF means the number of
L-phenylalaninol molecules formed per second per metallic
Acknowledgements
We would like to acknowledge the financial support from the
National Basic Research Program of China (2010CB732300),
the “ShuGuang” Project (10GG23) and the Leading Academic
Discipline Project (J51503) of Shanghai Municipal Education
Commission and Shanghai Education Development Foundation,
and The School to Introduce Talents Project (YJ2011-46).
2 3
copper atom in the boundary between CuO and ZnO or Al O .
The results are listed in Tables 1, 3 and 4. Fig. 15 shows the
relationships between B-TOF and dCuO (and SCu), like the rela-
tionships of TOF against dCuO (and SCu) in Fig. 14.
The results above show that B-TOF values are remarkably
larger than TOF values, as the amount of B-sites are less than
that of the surface active sites. For the particle size of CuO
Notes and references
(
dCuO), B-TOF decreases with a decrease in dCuO, that is to
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for the title reaction. For the surface area of Cu (SCu), B-TOF
0
decreases with an increase in SCu, which is similar to the
situation of TOF against dCuO and SCu. Unlike TOF, the
effects of dCuO and SCu on B-TOF are more obvious, which
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7
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. Conclusions
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In summary, the CuZn0.3AlO
x
(CZA) catalysts were prepared
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by different precipitation methods, and their physicochemical
properties are greatly affected by the preparation method and
conditions. The uniform size distribution of CuO species can
be obtained by fractional co-precipitation. The appropriate
aging time is 2 h, and the catalyst aged for 2 h has the largest
metallic copper surface area (SCu) and surface copper amount
and the smallest CuO crystallites. The lower calcination
temperature is favorable for increasing the surface area and
2 4
metallic copper surface area of the catalyst, and the CuAl O
spinel phase would form after calcination at 550 °C. The
catalytic hydrogenation activity of the Cu/ZnO/Al O catalysts
14 Z. Wang, Y. T. Cui, Z. B. Xu and J. Qu, J. Org. Chem., 2008,
73, 2270.
2
3
not only greatly depends on the metallic copper surface area
but also on the interaction between the metallic copper and
zinc oxides.
The Cu/ZnO/Al O catalyst (CZA-d-2-450) prepared by frac-
2 3
tional co-precipitation with aging at 70 °C for 2 h and calcina-
tion at 450 °C for 4 h shows the highest activity for
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catalyst is the largest (0.120 m
Cu/ZnO/Al catalysts. Over this catalyst, L-phenylalanine
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s
g ) among all of the
2
O
3
2
2
h, and 83.6% selectivity to L-phenylalaninol without racemi-
zation was achieved. Further investigations regarding the cata-
lytic reaction mechanism and deactivation and regeneration
of the catalysts are in progress and will be reported in due
2 3
course. Since the Cu/ZnO/Al O catalyst is very cheap and
easily prepared, this catalyst can be used widely in this selec-
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