Chemoselective hydrogenation of unsaturated nitro compounds to unsaturated amines by Ni-Sn alloy catalysts

Article abstract of DOI:10.1246/cl.180458

Ni-Sn alloy catalysts were prepared and applied to the hydrogenation of 4-nitrostyrene at 383423 K using H₂ gas as the hydrogen donor. Ni₃Sn₂ alloy showed a significantly high conversion and selectivity towards 4-aminostyrene (Conv. 100%, Sel. 99%). Various unsaturated nitro compounds were also successfully converted into their corresponding unsaturated amines.

Full text of DOI:10.1246/cl.180458

Advance Publication Cover Page
Chemoselective hydrogenation of unsaturated nitro compounds to unsaturated amines
by Ni-Sn alloy catalysts
Nobutaka Yamanaka, Takayoshi Hara, Nobuyuki Ichikuni, and Shogo Shimazu*
Advance Publication on the web June 21, 2018
doi:10.1246/cl.180458
© 2018 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
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Publication manuscripts.

1
Chemoselective hydrogenation of unsaturated nitro compounds to unsaturated amines
by Ni-Sn alloy catalysts
Nobutaka Yamanaka, Takayoshi Hara, Nobuyuki Ichikuni, and Shogo Shimazu*
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University,
1-33 Yayoi, Inage, Chiba 263-8522, Japan
E-mail: shimazu@faculty.chiba-u.jp
1
2
3
4
5
6
7
Ni-Sn alloy catalysts were prepared and applied to the
50 bimetallic strategies alter the electronic properties of the
51 active metals, leading to the improvement of the selectivity
52 towards nitro group. Since numerous SMSI effects have
53 been reported for the hydrogenation of nitroarenes, we focus
54 on the application of alloys. To date, Ni-Fe and Ni-B alloy
55 catalysts have been prepared and applied to the transfer
56 hydrogenation of various substituted nitroarenes, using
57 sodium borohydride and hydrazine hydrate as the hydrogen
58 donors, respectively.3,24,25 The reactions gave the
59 corresponding amines with remarkably high yields and
60 selectivities. Although these reducing agents are commonly
61 employed in laboratories, H2 gas is preferred in industry, as
62 it is inexpensive and only produces water as a by-product.26
hydrogenation of 4-nitrostyrene at 383-423 K using H2 gas
as the hydrogen donor. Ni3Sn2 alloy showed a significantly
high conversion and selectivity towards 4-aminostyrene
(Conv. 100%, Sel. 99%). Various unsaturated nitro
compounds were also successfully converted into their
corresponding unsaturated amines.
8
9
Keywords:
Chemoselective
hydrogenation,
Unsaturated nitro compounds, Ni-Sn alloy catalyst
10
The chemoselective hydrogenation from unsaturated
11 nitro compounds to unsaturated amines is an industrially
12 important and valuable process in the production of
13 agrochemicals, pharmaceuticals, dyes, and other high-value
14 fine chemicals.1-3 Typically, the Bechamp process, which
15 uses a zero valent iron and acid, has been employed to
16 reduce nitro group.4 This process has several drawbacks,
17 including a large amount of waste material (e.g., iron oxide
18 sludge and neutralization salts) and poor atom economy as it
19 is a stoichiometric reaction. As a result, it has been regarded
20 as an environmentally harmful process.3,5-7 Catalytic
21 hydrogenation of nitro group overcomes these
22 disadvantages. Recently, novel metals, such as Au and Ag,
23 showed notably high selectivity towards nitro group in the
24 hydrogenation of various substituted nitroarenes,
25 accompanied by low activity because they are intrinsically
26 poor capability for H2 activation.1,8-12 On the other hand, a
27 number of transition metals, such as Pt, Pd, Ru, and Ni,
28 have been reported as highly efficient metals for the
29 hydrogenation of nitrobenzene.1,5,8,13 These transition metals,
30 however, are well known to have low selectivity towards
31 nitro group when other reducible groups are present in the
32 substrate.14-18 This low selectivity towards nitro group
33 prevents the practical application of these catalysts. Of these
34 metals, Ni is the most inexpensive and would be a good
35 catalytic candidate from an economic and industrial point of
36 view.15,19 Therefore, the development of an environmentally
37 benign, cost-effective, and highly selective Ni-based
38 catalyst for the hydrogenation of various unsaturated nitro
39 compounds is highly desirable.
63
Previously, we prepared Ni-Sn alloy catalysts via
64 coprecipitation method and applied them to the
65 hydrogenation of furfural to furfuryl alcohol, using H2 gas
66 as the hydrogen donor.27 The synergetic effect between Ni
67 and Sn enhanced the chemoselectivity of the carbonyl group
68 up to 100%. These alloy catalysts had an ordered atom
69 arrangement, which could be controlled by varying the
70 molar feed ratios of Ni/Sn. In our ongoing research on this
71 catalyst, we wondered whether Ni-Sn alloy catalysts could
72 be applied to the hydrogenation of unsaturated nitro
73 compounds. Herein, the hydrogenation of unsaturated nitro
74 compounds was conducted under H2 atmosphere over Ni-Sn
75 alloy catalysts. After optimizing the reaction conditions, a
76 wide variety of unsaturated nitro compounds were
77 investigated for the substrate scope.
78
Ni-Sn(X) alloy catalysts (X = Ni/Sn molar ratio) were
79 prepared according to the previously reported procedure and
80 a detailed preparatory procedure is provided in the ESI.27
81 After the hydrothermal reaction at 423 K for 24 h, a gray
82 powder was obtained and denoted as Ni-Sn(X). The
83 subsequent H2 treatment at Y K for 1 h yielded a black
84 powder, which was denoted as Ni-Sn(X)HT-Y.
85
Powder X-ray diffractograms (XRD) were recorded on
86 a MiniFlex 600 Rigaku incident X-ray diffractometer using
87 CuK radiation (= 0.15418 nm) operated at 40 kV and 15
88 mA and identified against JCPDS-ICDD. The samples were
89 scanned at a rate of 0.02o step-1 over the range 20° ≤ 2≤
90 80° (scan time = 5 s step-1).
40
The aid of additives has been reported to enhance the
91
The catalytic hydrogenation was conducted in a high-
41 selectivity towards nitro group due to poisoning/blocking
42 the sites for undesired reactions.9,14 However, the addition of
43 soluble metal salts, such as iron salts with Pt-Pb/CaCO3 or
44 vanadium salts with Pt/C-H3PO4, affects the purity of the
45 target products.20,21 Compared to the above non-bimetallic
46 strategy, the modification of the active sites with a metal
47 oxide support or a second metal is more environmentally
48 benign and efficient, which is called as strong metal support
49 interaction (SMSI) or alloying, respectively.9,20,22,23 Both
92 pressure autoclave with a pressure gauge, a magnetic stirrer,
93 and an oil bath. The mixture of the substrate, catalyst,
94 solvent, and internal standard material was put into a glass
95 reaction tube and stirred at room temperature for a selected
96 time. The tube was placed into the autoclave. Then, pure
97 hydrogen gas was introduced and kept at a desired pressure,
98 and the reaction system was heated to a given temperature.
99 The reaction system was purged with H2 gas in order to

2
1
2
3
4
5
6
7
8
9
remove the air before heating up the solution. After the
reaction, the product was taken out of the reaction system
and analyzed by a gas chromatography. The products,
38 an increased reductive pretreatment temperature.20 Finally,
39 Ni-Sn alloy catalysts with various Ni/Sn molar ratios were
40 prepared and treated under H2 atmosphere at 673 K.
41 According to the XRD results, the observed Ni/Sn molar
42 ratios in the Ni-Sn alloy catalysts matched the feed ratios of
43 the precursors; Ni-Sn(3.0)HT673 and Ni-Sn(0.75)HT673
44 catalysts exhibited Ni3Sn and Ni3Sn4 alloy patterns,
45 respectively.28 The Ni-Sn particle sizes did not correlate
46 with the Ni/Sn molar ratios (Table S1, ESI).
expect for nitrobenzene, were analyzed by
chromatography (GC-8A, Shimadzu, using
a
gas
flame
a
ionization detector) equipped with a flexible quartz capillary
column coated with Silicon OV-17. On the other hand,
nitrobenzene was analyzed by the same gas chromatography
with Silicon OV-101. The amount of the products was
10 quantified based on the internal standard method using n-
47
The hydrogenation of 4-nitrostyrene was used as a
11 dodecane as an internal standard material.
48 model reaction in order to evaluate the catalytic
49 performance as the selective hydrogenation of nitro group is
50
12
We prepared Ni-Sn(1.5) catalysts without H2 treatment,
13 with H2 treatment at 623 K, 673 K, and 723 K (denoted as
14 Ni-Sn(1.5), Ni-Sn(1.5)HT623, Ni-Sn(1.5)HT673, and Ni-
15 Sn(1.5)HT723, respectively). The XRD pattern of Ni-
16 Sn(1.5) catalyst showed broad peaks at 2 = 30.6°, 43.2°,
17 and 44.2°, which were assigned to the (101), (102), and
18 (110) lattice planes of Ni3Sn2, respectively (Fig. 1a).28 After
19 H2 treatment at 673 K, sharp Ni3Sn2 alloy peaks were
20
51 Scheme 1 Hydrogenation pathway of 4-nitrostyrene
52
53 a challenging task when one or more reducible group is
54 present in the same molecule (Scheme 1).1,8,29 The results
55 are summarized in Table 1. The metallic Ni catalyst showed
56 excellent activity but poor selectivity towards 4-
57 aminostyrene, which was attributed to the intrinsic property
58 of metallic Ni (Table 1, entry 1). The conversion and
59 selectivity reached 79% and 60%, respectively, after 0.75 h.
60 The addition of electropositive Sn to Ni produced a
61 significant improvement in the chemoselectivity of nitro
62 group, which may be attributed to the synergistic effect
63 between the two metals. Ni-Sn(1.5) and Ni-Sn(1.5)HT673
64 catalysts gave 4-aminostyrene selectivity of 95% and 87%,
65 respectively, with full conversion of 4-nitrostyrene (Table 1,
66 entries 2 and 4). Although Ni gave 4-ethylnitrobenzene and
67
68 Table 1. Hydrogenation of 4-nitrostyrene with the
69 prepared catalysts
Conv.a Sel.b
Time
Entry
Catalyst
(%)
79
(%)
60
95
100
87
99
91
93
80
28
90
60
(h)
0.75
41
165
11
26
16
30
11
2
1
Ni
2
Ni-Sn(1.5)
100
90
3c
4
21 Figure 1. XRD patterns of Ni-Sn(1.5) catalysts (a) without
22 H2 treatment, (b) with H2 treatment at 723 K, (c) 673 K, and
23 (d) 623 K. (▼) Ni3Sn2.
Ni-Sn(1.5)HT673
100
100
93
5c
6d
7e
8
24
25 observed (Fig. 1c). Notably, no peaks from the original Ni
26 and Sn metals could be assigned, even without H2 treatment.
27 Using the Scherrer’s equation on the Ni3Sn2(101) peak at 2
28 = 30.6°, the Ni3Sn2 particle sizes of both Ni-Sn(1.5) and Ni-
29 Sn(1.5)HT673 catalysts were calculated to be 10.0 and 27.3
30 nm, respectively (Table S1, ESI). Then, several Ni-Sn(1.5)
31 catalysts were prepared by varying the H2 treatment
32 temperatures. The XRD results exhibited sharp Ni3Sn2 alloy
33 peaks for all prepared catalysts (Fig. 1). The Ni3Sn2 particle
34 sizes increased from 18.9 nm to 39.8 nm when the H2
35 treatment temperature was increased from 623 K to 723 K
36 (Table S1, ESI), which was consistent with a previous report
37 that Pt nanoparticle size in Pt/ZnO catalyst increased with
92
Ni-Sn(1.5)HT723
Ni-Sn(1.5)HT623
Ni-Sn(3.0)HT673
Ni-Sn(0.75)HT673
100
100
87
9
10
11
14
22
19
70 Reaction conditions: 4-nitrostyrene, 0.50 mmol; 4-
71 nitrostyrene/Ni molar ratio = 10; 1,4-dioxane, 5 mL; n-
72 dodecane, 0.30 mmol; H2, 3.0 MPa; 423 K. aThe conversion
73 was determined by GC using an internal standard technique.
c
d
74 bThe selectivity of 4-aminostyrene. 383 K. H2, 2.0 MPa.
75 eH2, 1.0 MPa.

3
1
2
3
4
5
6
7
8
9
4-ethylaniline as byproducts, Ni-Sn(1.5) catalysts clearly
suppressed the formation of 4-ethylnitrobenzene. This
suggests that nitro group is preferentially hydrogenated over
olefin group. This high chemoselectivity towards nitro
group may be caused by an interaction between the electron
lone pair of nitro group and the partially electropositive Sn.
In other words, -interaction between electron lone pair on
oxygen of nitro group and the Sn surface of the catalyst
plays an important role in the chemoselectivity.30 Here, we
41 that the activation of H2 is the rate-limiting step. Ni-
42 Sn(1.5)HT673 catalyst showed the highest activity when the
43 hydrogenation was conducted at a pressure of 3.0 MPa.
44
To determine the optimal H2 treatment temperature and
45 Ni/Sn molar ratio, several Ni-Sn alloy catalysts were
46 prepared and applied to the reaction (Table 1, entries 4 and
47 8-11), and Ni-Sn(1.5)HT673 catalyst showed the best
48 catalytic performance.
49
After optimizing the reaction conditions for the
10 can safely conclude that Ni3Sn2 alloy observed in the XRD
11 measurements contributed the high chemoselectivity in
12 hydrogenation of 4-nitrostyrene. In addition, H2 treatment is
13 crucial in order to obtain higher activity towards nitro group,
14 when compared to the hydrogenation results of Ni-Sn(1.5)
15 and Ni-Sn(1.5)HT673 catalysts.
50 hydrogenation of 4-nitrostyrene, the substrate scope of this
51 catalytic system was investigated. The catalyst with the best
52 performance, Ni-Sn(1.5)HT673 catalyst, was applied for the
53 hydrogenation of a variety of unsaturated nitro compounds.
54 The results are summarized in Table 2. From Table 2, Ni-
55 Sn(1.5)HT673 efficiently yielded the corresponding
56 unsaturated amines. The position of olefin group had little
57
16
To optimize the reaction conditions, at first, the
17 hydrogenation was conducted under different reaction
18 temperatures (Table 1, entries 2-5). By lowering the reaction
19 temperature from 423 K to 383 K, the selectivity was
20 somewhat enhanced for both Ni-Sn(1.5) and Ni-
21 Sn(1.5)HT673 catalysts, consistent with the report.31 Since
22 the reaction intermediates were slowly transformed to final
23 product at lower temperatures, the product concentration
24 became higher. This similar tendency was observed in the
25 previous report.31 At 383 K, Ni-Sn(1.5)HT673 catalyst
26 showed a selectivity of 99% towards 4-aminostyrene with
27 full conversion of 4-nitrostyrene after 26 h (Fig. 2).
28 However, the best catalytic activity was achieved at 423 K.
29
58 Table 2. Hydrogenation of various unsaturated nitro
59 compounds
100
4-Aminostyrene
90
4-Ethylnitrobenzene
80
4-Ethylaniline
70
60
50
40
30
20
10
0
60 Reaction conditions: Substrate/Ni molar ratio = 10; 1,4-
61 dioxane, 5 mL; n-dodecane, 0.30 mmol; H2, 3.0 MPa; 423 K.
62 aThe conversion was determined by GC using an internal
63 standard technique. bThe selectivity of unsaturated amines.
64
65 effect on the hydrogenation (Table 2, entry 1). The
66 selectivity of 3-aminostyrene reached 86% with full
67 conversion of 3-nitrostyrene after 10 h, which was similar to
68 the hydrogenation of 4-nitrostyrene. The hydrogenation of
69 4-nitrostilbene resulted in remarkably high selectivity,
70 which may be attributed to the steric hindrance from the two
71 benzene rings of 4-nitrostilbene (Table 2, entry 2).
72 Nitrobenzene, 1-nitronaphthalene, and 5-nitroindol were
73 also successfully hydrogenated with excellent conversion
74 and selectivity, as it was difficult to hydrogenate the olefin
75 group inside five- or six-membered rings due to the
76 aromatic property (Table 2, entries 3-5). No side products
77 were detected by GC. Although halogen-substituted
78 nitrobenzene tends to dehalogenate, in this study, 4-
79 chloronitrobenzene was completely hydrogenated into the
0
10
20
Time / h
30 Figure 2. Time profile of the hydrogenation of 4-
31 nitrostyrene with Ni-Sn(1.5)HT673 catalyst at 383 K.
32 Reaction conditions: 4-nitrostyrene, 0.50 mmol; 4-
33 nitrostyrene/Ni molar ratio = 10; 1,4-dioxane, 5 mL; n-
34 dodecane, 0.30 mmol; H2, 3.0 MPa; 383 K.
35
36
Then, we investigated the effect of H2 pressure on the
37 catalytic performance (Table 1, entries 4, 6, and 7). As seen
38 in the results, as hydrogen pressure increased, the reaction
39 time became shorter, and the selectivity decreased. The
40 dependence of the reaction time on the H2 pressure suggests

4
62 14. L. Wang, J. Zhang, H. Wang, Y. Shao, X. Liu, Y.-Q. Wang, J. P.
1
2
3
4
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8
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the C-X bond (Table 2, entry 6).32-34 Hydrogenation of 4-
nitrophenol proceeded almost quantitatively (Table 2. entry
7). Finally, -COOCH3 and -COCH3 groups showed high
tolerance and their corresponding unsaturated amines were
obtained with significantly high selectively (Table 2, entries
8 and 9). The catalytic system developed here is an
attractive approach for the catalytic hydrogenation of
unsaturated nitro compounds, even if they contain other
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