A continuous process for the reductive deoxygenation of aromatic ketones over Cu30Cr10/γ-Al2O3

Article abstract of DOI:10.1007/s11164-011-0454-0

A continuous process for reduction of aromatic ketones to the corresponding aromatic alkanes over Cu₃₀Cr₁₀/γ-Al ₂O₃ was established in a fixed-bed reactor. A series of aromatic ketones were examined, and all of the corresponding aromatic hydrocarbons were obtained in more than 95% yield. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and H ₂-temperature programmed reduction. Cu⁰ is believed to be the active center of the catalyst. Cr was found to have an important effect on catalytic activity and stability. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0454-0

Res Chem Intermed (2012) 38:1341–1349
DOI 10.1007/s11164-011-0454-0
A continuous process for the reductive deoxygenation
of aromatic ketones over Cu30Cr10/c-Al2O3
•
Jianchao Ma Shuai Liu Xiangjin Kong
•
•
•
Xiaopeng Fan Xilong Yan Ligong Chen
•
Received: 20 November 2011 / Accepted: 5 December 2011 / Published online: 21 December 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A continuous process for reduction of aromatic ketones to the corre-
sponding aromatic alkanes over Cu30Cr10/c-Al2O3 was established in a fixed-bed
reactor. A series of aromatic ketones were examined, and all of the corresponding
aromatic hydrocarbons were obtained in more than 95% yield. The catalysts
were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and
H2-temperature programmed reduction. Cu0 is believed to be the active center of the
catalyst. Cr was found to have an important effect on catalytic activity and stability.
Keywords Reductive deoxygenation Á Aromatic ketones Á Aromatic alkanes Á
Cu30Cr10/c-Al2O3
Introduction
Reduction of the benzyl carbonyl group of aromatic ketones to aromatic alkanes is
an important reaction in the fine chemical industry. It is of special significance in the
reduction of chemicals from the F–C acylation reaction to obtain long-chain
alkanes. As an example, this reaction has been applied to the synthesis of idebenone
[1, 2]. The Clemmensen reaction (acidic) [3] and the Wolff–Kishner–Huangminlon
reaction (basic) [4] are traditional procedures used to complete this transformation,
but these two methods have a deleterious effect on the environment [5, 6].
Nowadays, heterogeneous catalysts, for example nickel [7–9], copper [10], platinum
J. Ma Á S. Liu Á X. Fan Á X. Yan Á L. Chen (&)
School of Chemical Engineering and Technology,
Tianjin University, Tianjin 300072, People’s Republic of China
e-mail: lgchen@tju.edu.cn
X. Kong
School of Pharmaceutical Science and Technology, Tianjin University,
Tianjin 300072, People’s Republic of China
123

1342
J. Ma et al.
[11, 12], and palladium [13, 14]-based catalysts, are frequently used for
hydrodeoxygenation of aromatic ketones in an autoclave. In our previous work
[15], a continuous process was established for reduction of benzaldehyde to toluene
over Ni30/c-Al2O3 in the presence of aniline and H2. However, part of aniline was
transformed to cyclohexylamine, dicyclohexylamine, and N-cyclohexylaniline as
by-products, which was inconvenient for purification of products.
In this work, a continuous process in absence of aniline was studied for reduction
of aromatic ketones to the corresponding alkanes. A series of Cu–Cr catalysts were
prepared and evaluated for this reduction. The effects of doping Cu/c-Al2O3 with Cr
were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy
(XPS), and H2-temperature programmed reduction (H2-TPR).
Experimental
Materials and catalysts
Pseudo-boehmite was purchased from Jiangyan Chemical Auxiliary Factory
(Jiangyan, China). All commercially available reagents and solvents were used
without further purification.
All catalysts used in this study were prepared by the coprecipitation-kneading
method. For example, Cu30Cr10/c-Al2O3 was prepared as follows. Under mechan-
ical agitation, a mixture of 45.67 g Cu(NO3)2Á3H2O and 30.77 g Cr(NO3)3Á9H2O in
200 mL H2O, and a solution of 34.84 g sodium carbonate in 200 mL H2O were
added dropwise simultaneously to a beaker containing 200 mL H2O at 75 °C over
1 h and aged for another 1 h. The precipitate was then isolated by filtration, washed
with distilled water until the pH was 7–7.5, and then dried for 6 h at 120 °C. The
dried samples were kneaded vigorously with a mixture of 20.85 g pseudo-boehmite
and 50 mL 2 wt% nitric acid, followed by molding into a bar by use of an extruder.
After drying in air at 120 °C for 4 h, the bars were calcined at 500 °C for 4 h, then
activated at 250 °C in a stream of hydrogen (1.0 MPa) before use.
General procedure for catalytic reduction of aromatic ketones
Reduction of aromatic ketones was performed in a fixed-bed reactor with an inner
diameter of 15 mm and a length of 1,100 mm, which was charged of 18.0 g catalyst.
For example, catalytic reduction of hypnone was carried out over Cu30Cr10/c-Al2O3.
A 20 wt% solution of hypnone in 1,4-dioxane was introduced into the reactor at a
speed of 0.3 mL/min by means of a syringe pump. The hydrogen pressure in
the reaction system was set by use of a hydrogen regulator. The temperature in the
reaction zone was measured by use of a thermocouple located in the center of the
tube and the thermocouple was connected to a proportion integration differentiation
(PID) cascade controller. The reaction mixtures were analyzed by gas chromatog-
raphy–mass spectrometry (GC–MS). GC–MS was performed at 70 eV on a HP 5971
mass-selective detector equipped with a HP 5890 gas chromatograph (Hewlett–
Packard). A SE-54 capillary column (30 m 9 0.25 mm i.d.) with 0.33 lm film
123

A continuous process for the reductive deoxygenation of aromatic ketones
1343
thickness (Lanzhou Zhongkekaidi Chemical New-tech, China) was used. The
column outlet was connected to a hydrogen flame ionization detector (FID). The
injection volume was 0.4 lL at a carrier gas flow of 2 mL/min helium and a split
ratio of 1:15. The injector temperature was 280 °C. The column temperature was
kept isothermal at 80 °C for 2 min, increased to 250 °C at 20 °/min, and maintained
at 250 °C for 10 min. The products were identified by comparing their retention
times and mass spectral patterns with those of authentic standards.
Catalyst characterization
The XRD patterns of these catalysts were recorded with a Rigaka D/max 2500 X-ray
diffractometer using Cu-Ka radiation (40 kV, 100 mA) in the range 10–80°. XPS
measurements were recorded with a PHI 1600 spectrometer using a Mg–Ka X-ray
excitation source. H2-TPR was measured using a micromeritics 2910 apparatus
equipped with a thermal conductivity detector (TCD).
Results and discussion
Catalyst selection and characterization
Because Ni30/c-Al2O3 was demonstrated to be effective for reduction of benzal-
dehyde to toluene in the presence of aniline in our previous work [15], we attempted
to carry out the reduction of hypnone over Ni30/c-Al2O3 in the absence of aniline
(Table 1). Unfortunately, the performance of this catalyst was disappointing in
respect of yield of phenylethane. This situation encouraged us to seek a more
suitable catalyst for reduction of aromatic ketones to alkanes. Because it has been
reported that Cu has catalytic activity in the hydrogenation of the carbonyl group
[8, 16], Cu30/c-Al2O3 was used for reduction of hypnone. The results are
summarized in Table 1.
The catalytic performance of Cu30/c-Al2O3 was better than that of Ni30/c-Al2O3.
Unfortunately, the catalytic activity of Cu30/c-Al2O3 decreases quickly with time-
on-stream, and conversion of hypnone decreases from 97.5 to 76.62% in 30 h.
Table 1 Reduction of hypnone over Ni30/c-Al2O3 and Cu30-Cri/c-Al2O3 (i = 0, 5, 10 or 15)
Catalyst
Conv. (%)
Yield. (%)
Sel. (%)
Phenylethane
Ethylcyclohexane
a-Alcohol
Ni30/c-Al2O3
96.7
97.5
98.3
99.8
99.8
76.6
87.0
93.2
99.1
99.2
79.2
89.2
94.8
99.3
99.4
16.1
2.3
8.8
4.5
–
Cu30/c-Al2O3
–
–
–
–
Cu30Cr5/c-Al2O3
Cu30Cr10/c-Al2O3
Cu30Cr15/c-Al2O3
–
Reaction conditions: 200 °C, 2.0 MPa hydrogen, 0.3 mL/min (20 wt% hypnone in 1,4-dioxane)
123

1344
J. Ma et al.
Because it has been reported that addition of Cr can improve the lifetime of
Cu-based catalysts [16–18], different amounts of chromium were added to Cu30/
c-Al2O3 used for the reduction of hypnone. The results are summarized in Table 1.
It is obvious that addition of Cr to Cu30/c-Al2O3 effectively increases the conversion
of hypnone, and selectivity for and yield of phenylethane. It was also found that
Cu30/c-Al2O3 had excellent activity at a chromium loading of 10 wt%, and that the
activity then approximately leveled off with further increase of chromium loading.
Thus, Cu30Cr10/c-Al2O3 was selected as the catalyst of this reaction.
The reaction conditions, including reaction temperature and the hydrogen
pressure, were optimized. It was found that at 180 °C and 2.0 MPa conversion of
hypnone was 99.8% and that selectivity for phenylethane reached 99.4%. Thus,
180 °C and 2.0 MPa were selected as the reaction temperature and hydrogen
pressure. The stability of Cu30Cr10/c-Al2O3 was then examined with time-on-stream
for 200 h under the optimum reaction conditions and the results are shown in Fig. 1.
Obviously, no deactivation occurs during this period, and conversion of hypnone
and selectivity for phenylethane remain at approximately 99.8 and 99.4%,
respectively. These results reveal that Cu30Cr10/c-Al2O3 has excellent activity and
stability in the reduction of hypnone to phenylethane.
Reduction of other aromatic ketones
On the basis of the above work, Cu30Cr10/c-Al2O3 was used for catalytic reduction
of other aromatic ketones; the results are summarized in Table 2.
As is apparent from Table 2, all the aromatic ketones are reduced to the
corresponding aromatic alkane in more than 95% yield. Reduction of hypnone at
180 °C can proceed effectively with excellent conversion and selectivity (Entry 1).
110
108
106
104
102
100
98
110
108
106
104
102
100
98
Conversion of hypnone
Selectivity for phenylethane
96
96
94
94
92
92
90
90
0
50
100
150
200
Reaction time (h)
Fig. 1 Service life of Cu30Cr10/c-Al2O3. Reaction conditions: 180 °C, 2.0 MPa hydrogen, 0.3 mL/min
(20 wt% hypnone in 1,4-dioxane)
123

A continuous process for the reductive deoxygenation of aromatic ketones
1345
Table 2 Catalytic reduction of aromatic ketones over Cu30Cr10/c-Al2O3
Entry
Substrate
Product
Temp.
( °C)
Conv.
(%)
Sel.
(%)
Yield.
(%)
1
180
200
220
220
200
200
99.8
99.0
98.8
99.3
99.7
99.1
99.4
96.8
98.5
96.4
99.1
98.3
99.2
95.8
97.3
95.8
98.9
97.4
2
3
4
5
6
Reaction conditions: 2.0 MPa hydrogen, 0.3 mL/min (20 wt% aromatic ketone in 1,4-dioxane)
Similar conversion of propiophenone and selectivity for propylbenzene are
obtained at 200 °C (Entry 2), and reduction of butyrophenone and valerophenone
to butylbenzene and n-amylbenzene is achieved with good results at 220 °C
(Entries 3, 4). This implies that reaction temperature must increase with increasing
alkyl chain length. In addition, it is apparent that the donating substituent group on
the aromatic ring slightly affects the catalytic reduction results (Entries 5, 6).
Characterization of the catalysts
To further understand the catalyst and study the effects of chromium on Cu30/
c-Al2O3, these catalysts were analyzed by XRD, XPS, and H2-TPR.
123

1346
J. Ma et al.
XRD
The XRD patterns of the reduced Cu30/c-Al2O3, Cu30Cr5/c-Al2O3, Cu30Cr10/
c-Al2O3, and Cu30Cr15/c-Al2O3 are shown in Fig. 2. The typical diffraction lines at
approximately 43.3°, 50.4° and 74.1° in all the XRD patterns can be assigned to
crystals of elementary copper. Calculated from the (1 1 1) diffraction line (43.3°) on
the XRD patterns, by use of Scherrer equation, the average diameters of Cu crystals
are 46.7, 34.7, 28.7, and 27.8 nm for the reduced Cu30/c-Al2O3, Cu30Cr5/c-Al2O3,
Cu30Cr10/c-Al2O3, and Cu30Cr15/c-Al2O3, respectively. It was found that the mean
diameter of Cu0 decreased with increasing amount of Cr in the catalyst. This
indicates that the Cr added to Cu30/c-Al2O3 improves the dispersion of Cu0 so the
catalyst has more active centers. This may be the reason for the good catalytic
performance of Cu30Cr10/c-Al2O3 [16].
Additionally, new XRD peaks of CuCr2O4 are obtained from all Cu–Cr/c-Al2O3
catalysts when chromium is added. This indicates that the added chromium interacts
with the copper to form CuCr2O4, which prevents the copper crystals from sintering,
and prolongs the lifetime of the catalyst.
XPS
The XPS spectra of Cu30Cr10/c-Al2O3 are shown in Fig. 3. The curve for Cu 2p3/2
can be divided into two peaks by use of the Gauss curve fitting method. The binding
energy for Cu 2p3/2 in Cu30Cr10/c-Al2O3 at approximately 932.57 eV (Fig. 3b) is
assigned to Cu0, which indicates that the copper on the surface of the catalyst is
mainly reduced to Cu0. The weak signal of the Cu 2p3/2 al at 934.89 eV for the
# Cu
∀ γ-Al O3
*CuAl2O3
+CuCr2O4
#
#
2
#
#
∀
+
*
+
*
*
(d)
(c)
#
+
∀
*
*
*
+
#
#
#
#
+
*
∀
∀
#
#
+
*
(b)
#
*
*
*
(a)
0
10
20
30
40
50
60
70
80
90
Two-Theta (deg)
Fig. 2 The XRD patterns of the reduced (a) Cu30/c-Al2O3, (b) Cu30Cr5/c-Al2O3, (c) Cu30Cr10/c-Al2O3,
and (d) Cu30Cr15/c-Al2O3
123

A continuous process for the reductive deoxygenation of aromatic ketones
1347
5
4.0x10 (a)
5
3.5x10
5
3.0x10
5
2.5x10
5
2.0x10
5
1.5x10
5
1.0x10
4
5.0x10
0.0
1000
800
600
400
200
0
Binding energy (eV)
4
4
3.0x10
4.2x10
932.57eV
(b)
576.99 eV
(c)
Cr2p
4
4.0x10
Cu2p
4
4
2.8x10
3.8x10
934.89 eV
4
3.6x10
4
4
2.6x10
3.4x10
4
3.2x10
4
4
3.0x10
2.4x10
4
2.8x10
600
590
580
570
560
980
970
960
950
940
930
920
910
Binding energy (eV)
Binding energy (eV)
Fig. 3 XPS spectra of reduced Cu30Cr10/c-Al2O3
Gaussian peak indicates that copper is also present as Cu2?. The binding energy for
the Cr 2p3/2 level at 576.99 eV (Fig. 3c) reveals that Cr exists mainly as Cr3?. These
results are in agreement with the XRD results that CuCr2O4 is present in Cu30Cr10/
c-Al2O3.
H2-TPR
H2-TPR was carried out to evaluate the reducibility of copper species in the
unreduced catalysts; the results are illustrated in Fig. 4. The reduction peaks of the
unreduced Cu30/c-Al2O3, Cu30Cr5/c-Al2O3, Cu30Cr10/c-Al2O3, and Cu30Cr15/
c-Al2O3 are 293, 245, 231, and 228 °C, respectively. This indicates that increasing
the amount of chromium in Cu30/c-Al2O3 from zero to 10 wt% drastically promotes
the reduction of CuO. Moreover, further increasing the amount of Cr in Cu30Cr10/
c-Al2O3 shifts the reduction peak of CuO only slightly. These results agree well
with the XRD results, and also prove that a reduction temperature of 250 °C and use
of Cu30Cr10/c-Al2O3 as catalyst are suitable.
123

1348
J. Ma et al.
228oC
231oC
(d)
(c)
245 oC
293oC
(b)
(a)
0
200
400
600
800
1000
Temperature/ oC
Fig. 4 H2-TPR curves for the unreduced (a) Cu30/c-Al2O3, (b) Cu30Cr5/c-Al2O3, (c) Cu30Cr10/c-Al2O3,
and (d) Cu30Cr15/c-Al2O3
Conclusions
A continuous aniline-free process has been established for reduction of aromatic
ketones to the corresponding aromatic alkanes in a fixed-bed reactor. Cu30Cr10/
c-Al2O3 was used as catalyst for the reaction and afforded more than 95% yields of
aromatic alkanes. The catalyst investigated were characterized by XRD, XPS, and
H2-TPR. Cu0 is believed to be the active center. Addition of Cr was found to
promote the dispersion of Cu, and to improve catalyst activity and stability.
Acknowledgments Financial support was provided by the National Natural Science Foundation of
China (grant nos. 20706043 and 20976123).
References
1. M. Watanabe, I. Imada, EP 58057 (1982)
2. F.E. Chen, Z.R. Guo, Method of Organic Pharmaceutical Synthesis (Chinese Medicine Science and
Technology Press, Beijing, 1998), pp. 56–58
3. J.H. Brewster, J. Am. Chem. Soc. 76, 6361 (1954)
4. M.I. Huang, J. Am. Chem. Soc. 68, 2487 (1946)
5. J.B. Lambert, D.E. Marko, J. Am. Chem. Soc. 107, 7978 (1985)
6. L. Luchetti, V. Rosnati, J. Org. Chem. 56, 6836 (1991)
7. R.H. Mitchell, Y.-H. Lai, Tetrahedron Lett. 21, 2637 (1980)
8. F. Zaccheria, N. Ravasio, M. Ercoli, P. Allegrini, Tetrahedron Lett. 46, 7743 (2005)
9. D. Poondi, M.A. Vannice, J. Mol. Catal. A 124, 79 (1997)
10. M.A. Vannice, D. Poondi, J. Catal. 169, 166 (1997)
11. M. Lenarda, M. Casagrande, E. Moretti, L. Storaro, R. Frattini, P. Stefano, Catal. Lett. 114, 79 (2007)
12. C.-S. Chen, H.-W. Chen, Appl. Catal. A 260, 207 (2004)
´
13. D. Prochazkova, P. Zamostny, M. Bejblova, L. Cerveny, J. Cejka, Appl. Catal. A 332, 56 (2007)
´
´
´
´
´
123

A continuous process for the reductive deoxygenation of aromatic ketones
1349
14. L. Xing, X. Wang, C. Cheng, R. Zhu, B. Liu, Y.F. Hu, Tetrahedron 63, 9382 (2007)
15. S. Liu, X.P. Fan, X.L. Yan, X.B. Du, L.G. Chen, Appl. Catal. A 400, 99 (2011)
16. X.P. Fan, S. Liu, X.L. Yan, X.B. Du, L.G. Chen, Catal. Commun. 11, 960 (2010)
17. M. Sun, X.B. Du, H.B. Wang, Z.W. Wu, Y. Li, L.G. Chen, Catal. Lett. 141, 1703 (2011)
18. G.Y. Bai, L.G. Chen, Y. Li, X.L. Yan, F. He, P. Xing, T. Zeng, Appl. Catal. A 277, 253 (2004)
123