Bimetallic Zn and Hf on silica catalysts for the conversion of ethanol to 1,3-butadiene

Article abstract of DOI:10.1021/acscatal.5b00376

Silica-supported catalysts for the conversion of ethanol to 1,3-butadiene were investigated. The combination of Hf(IV) and Zn(II) resulted in a stable, active, and selective catalyst in which the Zn(II) effectively suppressed the dehydration activity of Hf(IV); the catalyst preparation method plays a crucial role. Using the crystalline Zn-silicate hemimorphite as an alternative Zn(II) source proved to be even more successful in suppressing ethanol dehydration.

Full text of DOI:10.1021/acscatal.5b00376

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Letter
Bimetallic Zn and Hf on silica catalysts for
the conversion of ethanol to 1,3-butadiene
Trees De Baerdemaeker, Mathias Feyen, Ulrich Mueller, Bilge Yilmaz, Feng-Shou
Xiao, Weiping Zhang, Toshiyuki Yokoi, Xinhe Bao, Hermann Gies, and Dirk E. De Vos
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00376 • Publication Date (Web): 24 Apr 2015
Downloaded from http://pubs.acs.org on May 3, 2015
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Bimetallic Zn and Hf on silica catalysts for the conversion of ethanol to
1,3-butadiene
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Trees De Baerdemaeker a, Mathias Feyen b, Ulrich Müller b, Bilge Yilmaz c, Feng-Shou Xiao d, Weiping
Zhang e, Toshiyuki Yokoi f, Xinhe Bao g, Hermann Gies h and Dirk E. De Vos a,*
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a Centre for Surface Chemistry and Catalysis, KU Leuven, 3001, Leuven, Belgium; b Process Research and
Chemical Engineering, BASF SE, 67056, Ludwigshafen, Germany; c Process Catalysts & Technologies, BASF
Corporation, Iselin, New Jersey 08830, United States; d Zhejiang University, 310028, Hangzhou, China;
e State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024, Dalian, China;
f Chemical Resources Laboratory, Tokyo Institute of Technology, 226-8503, Yokohama, Japan; g State Key
Laboratory of Catalysis, Dalian Institute of Chemical Physics, 116023, Dalian, China; h Institute of Geology,
Mineralogy and Geophysics, Ruhr-University Bochum, 44780, Bochum, Germany
Corresponding author: dirk.devos@biw.kuleuven.be
Abstract: Silica supported catalysts for the conversion of ethanol to 1,3-butadiene were investigated. The
combination of Hf(IV) and Zn(II) resulted in a stable, active and selective catalyst in which the Zn(II)
effectively suppressed the dehydration activity of Hf(IV); the catalyst preparation method plays a crucial
role. Using the crystalline Zn-silicate hemimorphite as an alternative Zn(II) source proved to be even more
successful in suppressing ethanol dehydration.
Keywords: 1,3-butadiene - ethanol – zinc silicate – hemimorphite – heterogeneous catalysis – silica
impregnation
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1,3-Butadiene (BD) is a frequently used building block in polymer chemistry. It is typically obtained as a
by-product from steam cracking where ethene and propene are the main products. However, the recent
changes to the use of a lighter feedstock for steam cracking and the increased usage of shale gas as an
alternative source for ethene have resulted in a price increase for BD.1 To meet the global demand for
BD, alternative synthesis routes that preferentially do not rely on fossil fuels are being investigated. One
of these routes that has received an increasing amount of attention over the past years is the conversion
of ethanol to BD.2-12 This reaction, which is also known as the Lebedev process, has already been
described in the early twentieth century using mixed metal oxides as catalysts, typically MgO/SiO2 or
ZnO/Al2O3.13-16
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The mechanism of the multistep reaction of ethanol to butadiene is complex and has not yet been fully
elucidated, in spite of large research efforts, e.g., in a recent publication by Chieregato et al.17
Nevertheless, there is a consensus on a number of key steps (Scheme 1): (1) the dehydrogenation of
ethanol to acetaldehyde catalyzed by basic or redox sites, (2) the aldol condensation of acetaldehyde, (3)
dehydration and a Meerwein-Ponndorf-Verley type reduction on basic or Lewis acid sites resulting in
crotyl alcohol, and (4) a final dehydration step to 1,3-BD, possibly on weak acid sites.4, 9, 18-22 The
acetaldol and crotonaldehyde products are not always observed, presumably due to their short lifetime
under reaction conditions.8 On the other hand, ethene and diethylether are frequently detected as side
products, resulting from reactions promoted by Brønsted acid sites.8, 23 The different types of catalysts
used for this reaction have recently been reviewed by Makshina et al.16 For instance, MgO/SiO2 systems,
as studied by Ohnishi et al.,24 or by Kvisle et al.,18 are a well-studied class of catalysts, which, for a long
time, had the highest reported butadiene selectivities (up to 87 %).24 However, the compositional
optimum seems narrow and catalyst stability is uncertain. Considerable improvements were realized by
adding Ni or Ag to the catalyst to facilitate the dehydrogenation.5, 12, 25 Such recent work shows that
correct tuning of the transition metal composition allows significant progress in comparison with the
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binary MgO/SiO2 systems. A related type of catalyst comprises a combination of transition metal oxides
deposited on a silica support without the use of MgO. This approach has for instance been investigated
by Jones et al.,4 where the highest selectivity to BD (67 % at 45 % ethanol conversion) was obtained using
a trimetallic combination of Cu(II), Zr(IV) and Zn(II) (1 wt% each). In the present work, a fumed silica
support was doped with different transition metal combinations; the catalytic function of each
compound in the conversion of ethanol to butadiene was investigated, and the influence of the
deposition method on catalyst performance was studied. Special attention was devoted to maximizing
the butadiene effluent concentration and to minimizing the loss of ethanol to by-products from acid-
catalyzed reactions like ethene and diethylether.
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Scheme 1. Generally accepted overall scheme for the formation of BD from ethanol.
As a starting point, the trimetallic Cu(II)-Zn(II)-Zr(IV) system of Jones et al.4 was taken (Table 1, entry 1,
Figure S1, S2). With this catalyst, a high selectivity of 61 % to BD at nearly full conversion was obtained,
which remained stable after 10 h of reaction in spite of a small decrease in conversion. Unfortunately,
this system still produces a significant amount of ethene (>10%) and related acid-catalyzed by-products.
Therefore, Hf(IV) was chosen as a softer metal to replace Zr. This resulted in a much lower ethene
production and an even higher selectivity to BD (Table 1, entry 2, Figure S3, S4, Table S1). By replacing
ZrO(NO3)2.H2O in the synthesis by HfCl4, the counteranion of the metal precursor salt was simultaneously
changed to chloride. To investigate the influence of the presence of chloride anions in the impregnation
slurry, the Zr(IV)-containing system was prepared using ZrCl4. This catalyst (Table 1, entry 3) shows a
higher stability and BD selectivity and a lower selectivity to ethene compared to the Cl-free system
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(Table 1, entry 1), indicating a beneficial effect of changing the metal source. However, the ethene
production is still higher than in the case of HfCl4, demonstrating the beneficial effect of the latter metal
on decreasing acid catalyzed by-product formation. With increasing Hf(IV)-content of the catalysts (Table
1, entries 2, 4, 5) the BD selectivity increases further at the expense of the acetaldehyde production.
Clearly, increasing the Hf(IV) content enhances the capacity of the catalyst for the aldol condensation
step and increases the BD-productivity.
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Table 1. Catalytic performance of trimetallic silica-supported catalysts. Catalytic results are shown after 0.5 h and 10 h reaction
at 360°C preceded by a 3 h reaction period at 300°C using a feed rate of 0.21 gEtOH gcat-1 h-1. Catalysts were prepared using
Cu(OAc)2, Zn(NO3)2.6H2O, ZrO(NO3)2.H2O and HfCl4 as metal precursors.
Selectivity (%)b
TOS
(h)
XEtOH
(%)
YieldBD
(%)
BD
(ppm)
=
=
=
Catalysta
C2
C3
Ac.
BD
61
C4
DEE
2.3
BuOH
BuO
Other
gBDgcat-1h-1
1
2
Cu1.0Zr1.0Zn0.5
0.5
98
79
96
95
96
91
99
96
99
99
16
15
2.8
2.2
2.8
2.8
2.4
2.3
3.7
3.4
3.4
3.1
5.8
8.4
9.8
11
6.6
5.3
6.0
4.0
5.3
4.8
2.6
2.2
6.4
5.5
0.49
0.50
0.50
0.62
0.41
0.44
0.62
0.36
0.28
0.30
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.27
4.8
5.3
7.9
8.7
6.3
7.3
12
59.5
0.074 16500
0.060 13400
0.077 17200
0.077 17300
0.079 17600
0.073 16300
0.065 14400
0.056 12400
0.090 19900
0.089 19800
10
0.5
10
61
65
65
66
64
52
46
72
71
2.7
1.3
1.1
2.1
2.1
0.5
0.4
1.0
1.1
48.0
61.8
61.9
63.3
58.4
51.8
44.6
71.6
71.1
Cu1.0Hf1.5Zn0.5
7.3
6.5
9.5
9.0
6.2
5.1
7.9
7.2
3c Cu1.0Zr1.5Zn0.5
0.5
10
7.9
10
4
5
Cu1.0Hf0.75Zn0.5
Cu1.0Hf3.0Zn0.5
0.5
10
22
29
14
0.5
10
2.6
4.2
6.4
7.2
0.16
aSubscript refers to the metal content (wt%). bSelectivity to ethene, propene, acetaldehyde, BD, butenes,
diethylether, butanol, butanal and other condensation products, respectively. cPrepared with ZrCl4.
To further investigate the role of each compound in the Cu(II)-Zn(II)-Hf(IV) system, mono- and bimetallic
catalysts were prepared (Table 2, Figure S5). If Hf(IV) is the only compound deposited on the silica
support (Table 2, entry 1), hardly any ethanol dehydrogenation takes place and ethene and diethylether
are the main products, resulting from acid catalysis. Adding Cu(II) to the catalyst alters the selectivity
entirely (Table 2, entry 2). In this case, there is sufficient dehydrogenation capacity to obtain a catalyst
with all required functionalities for the synthesis of butadiene. The ethene and diethylether formation is
correspondingly diminished but they are still produced in larger amounts than in the trimetallic system
(Table 1, entry 5). Additionally, the stability of the catalyst is much lower than in the Zn(II)-containing
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trimetallic system. On the other hand, deposition of only Zn(II) on the silica support results in a low
(<10%) selectivity to ethene and diethylether (Table 2, entry 3). A small amount of BD is formed but the
selectivity is low; most of the ethanol is converted to acetaldehyde without further condensation. This
clearly demonstrates the contribution of Zn(II) to the dehydrogenation capacity of the catalyst; unlike
Hf(IV), Zn(II) does not hydrolyze to form acid sites that give rise to large amounts of ethene or
diethylether. The combination of Zn, which strongly increases the ethanol dehydrogenation, and Hf(IV),
which catalyzes the subsequent aldol condensation finally results in a catalyst with a good BD selectivity
and a stable performance for many hours on stream (Table 2, entry 4, Table S1, entry 9). Remarkably, it
is important that Hf(IV) and Zn(II) are impregnated simultaneously on the same silica support: separate
impregnations of silica powders with these compounds, followed by physically mixing them together in a
mortar to obtain a catalyst with the same overall metal loading resulted in an inferior catalyst with lower
BD selectivites and a low stability (Table 2, entry 5). Clearly, both metals should be present in sufficient
proximity to each other in order to suppress the ethene and diethylether formation and to balance the
different catalytic functions resulting in BD formation.
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Table 2. Catalytic performance of mono- and bimetallic silica-supported catalysts. Catalytic results are shown after 0.5 h and 10
h reaction at 360°C preceded by a 3 h reaction period at 300°C using a feed rate of 0.64 gEtOH gcat-1 h-1. Catalysts were prepared
using Cu(OAc)2, Zn(NO3)2.6H2O, and HfCl4 as metal precursors.
Selectivityb
TOS
(h)
XEtOH
(%)
YieldBD
(%)
BD
(ppm)
=
=
=
Catalysta
Hf3.0
C2
C3
Ac.
1.0
BD
C4
DEE
17
BuOH
BuO
Other
gBDgcat-1h-1
1
0.5
48
43
99
75
66
67
72
68
60
34
82
0.1
0.0
2.2
1.0
1.2
1.0
2.0
1.9
1.5
1.1
0.4
0.1
49
25
14
12
63
60
36
29
0.1
0.1
7.7
3.0
0.4
0.2
4.1
4.0
5.1
2.9
<0.01
<0.01
0.27
0.17
2.5
<0.01
<0.01
<0.01
<0.01
1.3
0.0
0.0
2.8
2.0
12
0.2
0.001
<0.001
0.061
0.023
0.036
0.029
0.179
0.154
0.081
0.037
48
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0.5
10
82
29
52
10
0.6
3.2
5.1
57
62
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15
21
34
17
5.3
12
0.0
48.9
18.8
9.5
10
13600
5200
2c Cu1.0Hf3.0
3
4
Zn0.5
0.5
10
0.7
0.6
2.0
1.9
5.4
6.6
2640
7.5
7.7
7.0
26
2.6
1.2
13
7.6
2100
Hf3.0Zn0.5
0.5
10
2.6
0.37
0.50
1.5
6.0
6.5
2.9
3.0
45.3
41.0
21.6
9.8
12600
11400
6000
3.0
5d Hf3.0Zn0.5
0.5
10
0.32
0.46
21
2.5
2700
aSubscript refers to the metal content (wt%). bSelectivity to ethene, propene, acetaldehyde, BD, butenes,
diethylether, butanol, butanal and other condensation products, respectively. cFeed rate 0.21 gEtOH gcat-1 h-1. dAs a
1:1 mixture of silica containing 6.0 wt% Hf and silica containing 1.0 wt% Zn.
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To better understand the interaction between Hf(IV) and Zn(II), the zinc silicate hemimorphite (HM) was
used, both as support and as Zn(II)-source. Hemimorphite (Zn4Si2O7(OH)2.H2O) is a zinc silicate which, for
instance, can be used to catalyze the addition of methanol to propyne, using Zn2+ ions with open
coordination sites at the outer surface.26 In the conversion of ethanol to BD, it catalyzes the
dehydrogenation to acetaldehyde, but it hardly enables the subsequent reaction steps (Table 3, entry 1)
resulting in a similar performance as with the Zn(II)-on-silica catalyst (Table 2, entry 3). Using it as a
support for Hf(IV) barely improves the selectivity to BD but the ethene production is still strikingly low
(Table 3, entry 2). Remarkably, when HM is deposited onto silica, i.e. when it is used as a Zn(II) precursor
instead of the previously used Zn(NO3)2.6H2O, the observed selectivities change entirely. In a first
experiment, the HM was contacted with a Hf(IV)-containing silica support under reflux conditions (Table
3, entry 3). This already decreased the accumulation of acetaldehyde in the product stream and resulted
in an increased butadiene selectivity, but it also increased the ethene formation. Using this method of
combining Hf(IV) and Zn(II) on silica, the ethanol dehydration could not be sufficiently suppressed. On
the other hand, simply contacting both compounds (HM, Hf(IV) deposited on SiO2) in suspension at room
temperature largely suppresses the ethene formation (≈ 5 % selectivity) and, depending on the precise
composition, results in a stable BD selectivity of 70 % at nearly full ethanol conversion (Table 3, entries 4-
6).
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Table 3. Catalytic performance of HM-containing silica-supported catalysts (HM = hemimorphite). Catalytic results are shown
after 0.5 h and 10 h reaction at 360°C preceded by a 3 h reaction period at 300°C using a feed rate of 0.64 gEtOH gcat-1 h-1.
Catalysts were prepared using HfCl4 as metal precursor and HM as the sole Zn source.
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Selectivityb
TOS
(h)
XEtOH
(%)
YieldBD
(%)
=
=
=
Catalysta
1c HM
C2
C3
Ac.
BD
C4
DEE
0.2
BuOH
BuO
Other
gBDgcat-1h-1
BD (ppm)
0.5
76.2 4.6
78.3 3.9
75.6 4.3
77.3 3.8
2.0
1.6
1.4
1.2
11
60
62
5.5
4.3
12
8.8
39
18
66
61
71
67
70
69
0.1
0.1
0.2
0.2
27
3.1
3.3
1.1
0.95
1.1
24
24
4.2
0.016
0.013
0.033
0.025
0.141
0.066
0.217
0.208
0.264
0.245
0.260
0.257
1170
950
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13
14
15
16
17
18
19
20
21
22
23
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25
26
27
28
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10
0.5
10
0.2
0.6
0.7
0.2
0.1
0.8
0.9
0.7
1.1
1.4
1.3
3.4
8.7
2c Hf3.0 on HM
3d Hf3.2Zn5.1
4e Hf3.2Zn5.1
5e Hf2.5Zn16
6e Hf3.0Zn9.3
60
3.2
18
2400
61
3.6
1.1
19
6.8
1900
0.5
10
97.6
99.4
18
16
0.8
0.3
9.2
11
<0.01
0.23
0.41
0.79
0.42
0.48
0.06
0.26
0.45
0.83
2.8
4.6
20
37.5
17.5
57.7
55.3
70.2
65.2
69.3
68.4
10500
4900
14
32
0.5
10
88.1 5.3
90.6 4.1
99.2 5.8
97.3 4.9
2.6
2.5
3.6
3.6
3.6
3.3
3.2
3.3
3.8
4.3
5.0
5.2
11
16100
15400
19500
18200
19300
19100
3.3
13
0.5
10
4.0
6.5
2.4
3.3
2.5
8.1
9.4
5.4
7.4
3.2
0.5
10
98.8
10
1.8
98.6 7.5
2.3
aSubscript refers to the metal content (wt%). bSelectivity to ethene, propene, acetaldehyde, BD, butenes,
diethylether, butanol, butanal and other condensation products, respectively. cHM as support, no silica. dThe
appropriate amount of Hf was first impregnated onto the silica support and then contacted with HM in suspension
under reflux conditions for 3 h. eThe appropriate amount of Hf was first impregnated onto the silica support and
then contacted with HM in suspension at room temperature for 0.5 h.
Characterization of the catalysts with FTIR using pyridine as probe molecule provided further insights
into the catalyst performance (Table 4, Figure S6). The impregnation of Hf(IV) onto silica clearly
introduced Brønsted acidity (Table 4, entry 1), but this was significantly suppressed by the presence of
Zn(II) – either introduced as Zn(NO3)2.6H2O, or admixed using hemimorphite as the Zn(II) source (Table 4,
entries 3, 7-9). Adding Cu(II) to the catalyst only had a minor influence on the amount and type of acid
sites (Table 4, entries 3, 5) . Catalysts consisting only of Zn2+ and silica (Table 4, entries 2, 6) show an
almost negligible Brønsted acidity and correspondingly low ethene formation. While ZnO has been
described by Tanabe as being amphoteric,27 various studies have shown that in the interaction with
alcohols, ZnO primarily acts as a basic material.28-30 For materials loaded with both Hf(IV) and Zn(II), it is
therefore well conceivable that the Brønsted acidity generated by hydrolysis on Hf(IV), is neutralized by
neighbouring O-Zn groups, either on well-dispersed ZnO, or as Si-O-Zn moieties at the hemimorphite
surface. Remarkably, the mixture of Hf(IV) and Zn(II), impregnated separately onto silica (Table 4, entry
7
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4) has a limited number of Brønsted acid sites but still produces a significant amount of ethene (26 %
selectivity). This again demonstrates the significance of the impregnation procedure to balance not only
the amounts, but also the proximity to one another of the different catalytic functions.
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Table 4. Acid properties of selected catalysts determined via FTIR spectroscopy using pyridine as probe molecule. Catalytic data
are given for comparison.
BAS
LAS
=
Entry
Catalyst
1b Hf3.0
(μmol/g)a
(μmol/g)a
L/B
Y(BD,%)
0.2
S(BD,%)
0.4
S(C2 ,%)
5.0
47.3
9.4
82
2b Zn0.5
<0.1
2.0
13.9
67.9
21.3
53.0
13.0
40.6
44.1
25.5
-
34.4
18.1
36.2
-
7.1
45.3
21.6
71.6
4.2
9.5
63
36
72
5.5
66
71
70
6.6
7.7
26
3b Hf3.0Zn0.5
4b,c Hf3.0Zn0.5
5b Cu1.0Hf3.0Zn0.5
6d HM
7d Hf3.2Zn5.1
8d Hf2.5Zn16
9d Hf3.0Zn9.3
1.2
1.5
7.9
4.6
5.3
5.8
10
<0.1
0.7
54.5
-
57.7
70.2
69.3
<0.1
<0.1
-
aAmount of pyridine adsorbed on Brønsted acid sites (BAS) and Lewis acid sites (LAS) at 150°C as determined from
the IR absorption band of chemisorbed pyridine. bFrom impregnation of the appropriate amount of Cu(OAc)2,
Zn(NO3)2.6H2O and HfCl4 on silica. cAs a 1:1 mixture of silica containing 6.0 wt% Hf and silica containing 1.0 wt% Zn.
dThe appropriate amount of Hf was first impregnated onto the silica support and then contacted with HM in
suspension at room temperature for 0.5h.
While suppression of Brønsted acidity is clearly beneficial for the BD selectivity, Lewis acid sites are
needed both for the aldol condensation and for the Meerwein type reduction of the putative
crotonaldehyde intermediate. For the ethanol dehydrogenation, Zn-O moieties are effective, but for the
aldol reaction the Zn(II) centres are nearly inactive. In contrast, Hf(IV) is not only effective for the aldol
condensation, but it is also well known to promote hydrogen transfer reactions in MPV reactions. In the
latter, Hf(IV) is even more active than Zr(IV), which has at least partly been ascribed to the facile
displacement of –OH groups on the Hf(IV) by incoming alcohol reactants, at least for MPV reactions in
mild conditions.31
With the best HM-based catalysts of the present study, the ethene selectivity is 10% or even significantly
lower. While hardly any Brønsted acidity could be detected on these materials using the pyridine probe
method, it is probable that some of the water, produced during the reaction, causes a mild hydrolysis of
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the Hf(IV). This generates Brønsted acidity under reaction conditions, which is the likely cause for the
observed, limited ethanol dehydration.
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In summary, we demonstrated that bimetallic mixtures of Hf(IV)-Zn(II) and trimetallic mixtures of Cu(II)-
Hf(IV)-Zn(II) and Cu(II)-Zr(IV)-Zn(II) give remarkably active, selective and stable catalysts for the ethanol
to butadiene reaction. In these trimetallic catalysts, Cu(II) acts as a dehydrogenation catalyst. However,
the synthesis of bimetallic catalysts has shown that this function can also be performed by Zn(II), which,
in addition, also increases the stability of the catalysts. Hf(IV) is preferred over Zr(IV), especially in order
to avoid the competitive ethanol dehydration. Adding Zn(II) to the Hf(IV) containing catalyst is crucial to
obtain dehydrogenation capacity and to force the Hf(IV) into catalyzing the condensation of
acetaldehyde rather than the ethanol dehydration. In order to achieve this high BD selectivity, the
deposition method is crucial. The use of hemimorphite as Zn(II) source was particularly successful in
suppressing the ethene formation while simultaneously producing high concentrations of BD.
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Acknowledgement
T.D.B. is grateful for a PhD fellowship from F.W.O. - Vlaanderen (Research Foundation Flanders). The
authors acknowledge the support from BASF SE under the INCOE framework. We thank Sabina Accardo
for technical assistance.
Supporting information: experimental procedures, SEM, EDX, EPR, FTIR characterization, TG analysis.
This information is available free of charge via the Internet at http://pubs.acs.org/.
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TOC graphic
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-H2
-H2O
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ZnO + HfOx
on SiO2
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HfOx
on SiO2
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