Heterogeneously catalyzed oxidation of butanediols in base free aqueous media

Article abstract of DOI:10.1016/j.tet.2014.02.043

The oxidation of four butanediols under base-free conditions has been investigated using a set of Au, Pd and Pt catalysts prepared using sol-immobilization. The supported nanoparticles are found to be active with bimetallic alloys having much higher activity when compared with the monometallic counterparts. In general the AuPt catalysts are the most active and in all cases the corresponding C4 oxidation products were observed with high selectivity; sequential reaction of these products leads to the formation of acetic acid as an undesired by-product.

Full text of DOI:10.1016/j.tet.2014.02.043

Tetrahedron xxx (2014) 1e4
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Heterogeneously catalyzed oxidation of butanediols in base free
aqueous media
Yulia Ryabenkova, Peter J. Miedziak, David W. Knight, Stuart H. Taylor,
*
Graham J. Hutchings
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of four butanediols under base-free conditions has been investigated using a set of Au, Pd
and Pt catalysts prepared using sol-immobilization. The supported nanoparticles are found to be active
with bimetallic alloys having much higher activity when compared with the monometallic counterparts.
In general the AuPt catalysts are the most active and in all cases the corresponding C4 oxidation products
were observed with high selectivity; sequential reaction of these products leads to the formation of
acetic acid as an undesired by-product.
Received 3 December 2013
Received in revised form 12 February 2014
Accepted 14 February 2014
Available online xxx
Keywords:
Butanediol
Ó 2014 Published by Elsevier Ltd.
Catalytic oxidation
Gold catalysis
1. Introduction
Homogeneous catalysts have been successfully used21 but the dif-
ficulty of catalyst recovery and reuse has to be overcome. The dis-
Alcohol oxidation is a crucial process both for the synthesis of
fine chemical and important intermediates. Hence short chain al-
cohols, e.g., ethanol, 1-propanol and 2-propanol, can be catalyti-
cally oxidized to their corresponding acids, aldehydes or ketones
advantages displayed by these previous studies can be solved by
utilizing supported gold-containing nanoparticles under mild,
base-free conditions using water as a solvent and oxygen at low
pressures as the oxidant. The feasibility and applicability of all these
requirements will be evaluated in this paper.
1
e3
using heterogeneous precious metal supported catalysts.
Diols,
such as propane-1,2-diol or ethane-1,2-diol, can in turn be trans-
4
5
formed into hydroxyl-acids or dicarboxylic acids. In these cata-
lytic transformations the control of the selectivity is the important
2. Results and discussion
factor. We have recently investigated the possibility of oxidation of
propylene glycol6
e8
2.1. Oxidation of 1,2-butanediol
into lactic acid using supported gold-
containing supported nanoparticles and we now aim to extend
the use of these catalysts and examine the possibilities of oxidizing
the full range of C -diols over such catalysts.
4
In aqueous media 1,2-butanediol was oxidized mainly to 2-
hydroxybutyric acid and 1-hydroxy-2-butanone (see Scheme 1)
together with lesser amounts of lower carbon number products.
The oxidation of 1,2-butanediol was carried out in the absence
of base using mono- and bimetallic Au, Pd and Pt catalysts sup-
ported on activated carbon. The catalytic activity of these and the
formation of reaction products have been evaluated with the re-
sults reported in Table 1. In these studies the amount of metal has
been maintained at 1 wt %. The monometallic catalysts were less
active than the bimetallic alloy catalysts. Except for the supported
AuPt catalyst the effect of alloying was not particularly marked,
although all the bimetallic combinations were more effective as
catalysts than their monometallic counterparts. Regarding the
product distribution, clearly the monometallic Pt (Table 1, entry 2)
and Pt-containing bimetallic catalysts (Table 1, entries 4 and 5)
produce more acetic acid, whereas monometallic Au was the most
The oxidation of butanediols has been well studied previously.
Butanediols can be oxidized selectively to their corresponding
hydroxy-acids and hydroxy-ketones by a wide range of methods,
9
e11
12
including: electro-oxidation,
homogeneous catalysis, oxida-
13
tion in the gas phase using a heterogeneous catalyst and bacterial
oxidation.14 Many studies report high conversion and selectivity,
but these often utilize relatively harsh reaction conditions. For in-
2 2
stance, oxidants other than oxygen have been used including H O ,
1
5,16
and pyridinium bromochromate.
In addition, solvents, such as
17e20
acetonitrile, and alkaline or acidic media have been used.
*
Corresponding author. E-mail address: hutch@cf.ac.uk (G.J. Hutchings).
0
040-4020/$ e see front matter Ó 2014 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tet.2014.02.043
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2
Y. Ryabenkova et al. / Tetrahedron xxx (2014) 1e4
O
O
O
[O]
OH
OH
+
+
+
OH
OH
OH
OH
O
OH
Scheme 1. General scheme of 1,2-butanediol oxidation.
Table 1
Catalytic oxidation of 1,2-butanediol
Entry
Catalyst
Conversion
%)
Selectivity, %
(
2-Hydroxybutyric acid
1-Hydroxy-2-butanone
Propionic acid
Acetic acid
1
2
3
4
5
6
1%Pd
1%Pt
1%Au
0.5%Auþ0/5%Pt
0.5%Pdþ0/5%Pt
0.5%Auþ0/5%Pd
16
25
19
75
31
33
52
53
55
45
54
45
43
42
45
42
28
46
3
2
0
5
11
6
3
4
0
8
7
3
Reaction conditions: 0.6 M 1,2-butanediol, butanediol/total metal ratio¼2000, T¼100 ꢀC, P¼3 bar, time¼24 h, stirrer speed¼1000 rpm.
selective towards the hydroxy-acid and the hydroxy-ketone, with
small amounts of acetic and propionic acids also being formed.
Interestingly, the product distributions we observe in this work
differs from those reported previously. Catalytic oxidation over
ceria-supported AuPd catalysts resulted in formation of hydroxy-
aldehyde with high selectivities, whereas electro-oxidation in
basic media leads to the formation of both hydroxy-ketone and
hydroxy-aldehyde.23 Comparing these earlier literature results
and the products reported in Table 1, it is clear that the reaction
pathways are different when using the carbon-supported nano-
particles, as oxidation proceeds further with the hydroxy-acid be-
ing formed.
acid and formic acid were observed. As in the case of 1,2-butanediol
(see Section 2.1), hydroxy-aldehydes in both cases were not ob-
served in the present study due to their further oxidation to the
corresponding acids under the reaction conditions.
The same catalysts, previously tested for oxidation of 1,2-
butanediol, were used for oxidation of 1,3-butanediol and the re-
sults are shown in Table 2.
The 1%Au catalyst was very selective to 4-hydroxy-2-butanone,
but this was at low conversion. Surprisingly, monometallic 1%Pt/C,
is very active for 1,3-butanediol oxidation, and it gave the un-
saturated aldehyde 2-butenal (crotonaldehyde) as a by-product
(Table 2, entry 2), which probably results from the initial forma-
tion of 3-hydroxybutanal, which could then undergo dehydration
over the Lewis acid sites of the catalyst. This was an unexpected
result as 2-butenal would be assumed to be more reactive than 4-
hyrdoxy butyric acid, interestingly this result was also repeated in
entry 5 when the catalyst also contained platinum, suggesting the
presence of platinum can lead to stabilization of this product. In-
2
2
2
.2. Oxidation of 1,3-butanediol
The major products we observed from the oxidation of 1,3-
butanediol were 3-hydroxy-butyric acid and 4-hydroxy-2-
butanone (Table 2).
Table 2
Catalytic oxidation of 1,3-butanediol
Entry
Catalyst
Conversion (%)
Selectivity, %
-Hydroxy butyric acid
3
4-Hydroxy-2-butanone
2-Butenal
Acetic acid
1
2
3
4
5
6
1%Pd
1%Pt
1%Au
0.5%Auþ0/5%Pt
0.5%Pdþ0/5%Pt
0.5%Auþ0/5%Pd
9
35
7
67
54
34
47
42
0
56
54
35
53
10
100
39
23
63
0
47
0
3
18
0
0
1
0
2
5
3
Reaction conditions: 0.6 M 1,3-butanediol, butanediol/total metal ratio¼2000, T¼100 ꢀC, P¼3 bar, time¼24 h, stirrer speed¼1000 rpm.
Catalytic oxidation of 1,3-butanediol has already been re-
terestingly, the formation of crotonaldehyde is observed only for
Pt-containing catalysts, with the selectivity to 2-butenal decreasing
with the trend Pt>PdPt>AuPt. It is also apparent that the bimetallic
catalysts are more active than monometallic catalysts, with the
most active being 0.5%Auþ0.5%Pt/C. This demonstrates the syner-
gistic effect of the two metals on the oxidation activity.
16
ported using tungsten-containing mesoporous silica catalysts, in
the presence of 1,2-benzenediol as an auxiliary additive, over
ꢀ
a temperature range of 35e80 C for 1e16 h. Interestingly, the
products detected only partially correspond to those expected from
Scheme 2. 4-Hydroxy-2-butanone, 3-hydroxy-4-butanal, acetic
O
[
O]
OH
OH
OH
+
+
+
OH
OH
OH
O
O
O
Scheme 2. General scheme of 1,3-butanediol oxidation.
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Y. Ryabenkova et al. / Tetrahedron xxx (2014) 1e4
3
2
.3. Oxidation of 1,4-butanediol
butanedione were also observed along with acetic acid,
in this case the acetic acid could be formed by a retro-aldol
reaction. The monometallic catalysts all displayed similar
activity, which was enhanced by alloying two metals, with
the combination of AuPt being most active. The highest selec-
tivities to the hydroxy-ketone were detected when using mono-
metallic catalysts, though the conversion did not exceed 13%
The products that were detected from 1,4-butanediol oxidation
are shown in Scheme 3. One of the products formed is succinic acid,
the result of the oxidation of both hydroxyl groups. At present this
material is produced from a liquefied petroleum gas or petroleum
2
4
oil and new ways of its synthesis are of interest.
Scheme 3. General scheme of 1,4-butanediol oxidation.
As can be seen from Table 3,
g-butyrolactone is the major
Table 4
product formed in this reaction and this is largely independent of
the catalyst used. Its formation can be explained by the dehydrative
cyclization of 4-hydroxybutyric acid. Another possible pathway
could involve formation of g-butyrolactone from the aldehyde via
the cyclic hemiacetal, which is then oxidized. Remarkably, the 0.5%
Catalytic oxidation of 2,3-butanediol
Entry Catalyst
Conversion (%) Selectivity, %
Butanedione 3-Hydroxy- Acetic
2
-butanone acid
1
2
3
4
5
6
1%Pd
1%Pt
1%Au
0.5%Auþ0/5%Pt 62
0.5%Pdþ0/5%Pt 37
0.5%Auþ0/5%Pd 54
13
10
12
4
6
4
4
4
6
92
94
88
84
89
86
4
0
8
12
7
8
Auþ0.5%Pt/C catalyst fully converted 1,4-butanediol with selectiv-
ity to succinic acid (16%) 4-hydroxybutytic acid (21%) and
g-
butyrolactone (63%) (Table 3, entry 4). 0.5%Pdþ0.5%Pt/C was the
next most active catalyst (Table 3, entry 5), giving ca. 73% conver-
sion with 11% selectivity to succinic acid. Monometallic 1%Au/C
catalyst was most selective to the lactone at low conversion, but no
succinic acid was observed, probably as a result of the low con-
version obtained with this catalyst (Table 3, entry 3). We consider
that by further optimization of the reaction conditions it may be
possible to achieve higher yields of succinic acid.
Reaction conditions: 0.6 M 2,3-butanediol, butanediol/total metal ratio¼2000,
ꢀ
T¼100 C, P¼3 bar, time¼24 h, stirrer speed¼1000 rpm.
and so this high selectivity is probably related to the low
conversion.
Evidence of sequential oxidation has been reported in the lit-
Table 3
23,25
erature,
but these oxidations were carried out in basic media
Catalytic oxidation of 1,4-butanediol
using Au and Pt electrodes. Therefore, it was decided to evaluate the
possibility of the consecutive oxidation of 3-hydroxy-2-butanone
to butanedione and to observe if acetic acid was also formed. A
series of experiments were conducted in which 3-hydroxy-2-
butanone and butanedione were oxidized, using the same condi-
Entry Catalyst
Conversion Selectivity, %
(%)
Succinic 4-Hydroxy-
g-Butyrolactone
acid
butytic acid
1
2
3
4
5
6
1%Pd
1%Pt
1%Au
11
16
4
4
6
0
16
11
9
31
18
29
21
23
22
65
76
71
63
66
69
ꢀ
tions as for the butanediols, for 24 h at 100 C. 3-Hydroxy-2-
butanone was observed to be oxidized to butanedione with 21%
conversion, giving both butanedione and acetic acid with selec-
tivities of 47% and 53% accordingly. Oxidation of butanedione led to
the formation of acetic acid with 100% selectivity and 53% con-
version, thus suggesting that acetic acid formation arises from
diketone oxidation. This data shows that initially only one OH-
group of 2,3-butanediol is oxidized to form 3-hydroxy-2-
butanone; this compound in turn is further oxidized to butane-
dione. The diketone formed is then further oxidized and coupling
with CeC bond cleavage, gives acetic acid as a by-product. Thus, the
results reported above favour the assumption that the reaction of
0.5%Auþ0/5%Pt 100
0.5%Pdþ0/5%Pt
0.5%Auþ0/5%Pd
73
54
Reaction conditions: 0.6 M 1,4-butanediol, butanediol/total metal ratio¼2000,
ꢀ
T¼100 C, P¼3 bar, time¼24 h, stirrer speed¼1000 rpm.
2
.4. Oxidation of 2,3-butanediol
The results of 2,3-butanediol oxidation are shown in Scheme 4.
The main reaction products formed are butanedione and 3-
hydroxy-2-butanone.
2,3-butanediol oxidation over carbon supported catalysts in aque-
ous media can be considered sequential with the following rates of
oxidation: 2,3-butanedione>>3-hydroxy-2-butanone>2,3-butane
diol.
3. Conclusion
Scheme 4. General scheme of 2,3-butanediol oxidation.
The selective catalytic oxidation of the C4-diols has been in-
vestigated under mild base-free conditions using supported Au, Pd
and Pt supported monometallic and bimetallic nanoparticles. The
key feature of the reactions described in this paper is the absence of
undesirable oxidants and solvents. Water has been used as
Oxidation of 2,3-butanediol results in the formation of 3-
hydroxy-2-butanone as the major product and is not highly de-
pendent on the catalyst used (Table 4). Small amounts of
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4
Y. Ryabenkova et al. / Tetrahedron xxx (2014) 1e4
a reaction media together with oxygen at low pressures, making
the reaction conditions and the reagents relatively facile to handle.
Introducing the second metal to a monometallic catalyst enhances
the conversion due to synergistic effects, with the 1 wt % AuPt/C
being the most efficient in terms of both conversion and selectivity
for the oxidation of all butanediol homologues.
NMR spectroscopy was used for product identification; spectra
were acquired over a 16 scan period using a Bruker 400 MHz DPX
system with a 5 mm auto tune broadband probe. All samples were
2
prepared as dilute solutions in D O. Carbon mass balances were
calculated and were between 96 and 104%. Blank reactions have
also been carried out with no oxidation activity detected in the
absence of catalyst or with the KB-B carbon support.
4
4
. Experimental section
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weight average molecular weight MW¼9000e10,000 g mol
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8
7
4
4
8
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7
copy. The catalysts comprise nanoparticles in the size range
1
2
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6
1
2
1
2
4
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mixture was heated to 100 C for 24 h under constant stirring
1
(
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Please cite this article in press as: Ryabenkova, Y.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.02.043