Oxidative esterification of homologous 1,3-propanediols

Article abstract of DOI:10.1007/s10562-012-0872-7

The oxidative esterification of a homologous series of diols (1,3-propanediol,2-methyl-propanediol and 2,2-dimethyl-1,3-propanediol) with methanol has been investigated using titania-supported gold, palladium and gold-palladium catalysts using molecular oxygen. The gold-palladium catalysts showed the highest activity and 1,3-propanediol was the most reactive while the additional methyl groups decreased the reactivity. However, it is possible to achieve high selectivity to methyl 3-hydroxypropionate and 2-methyl-3- hydroxyisobutyrate by mono-oxidations. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-012-0872-7

Catal Lett (2012) 142:1114–1120
DOI 10.1007/s10562-012-0872-7
Oxidative Esterification of Homologous 1,3-Propanediols
Tatyana Kotionova
•
Christopher Lee
Albert F. Carley David J. Morgan
Graham J. Hutchings
•
Peter J. Miedziak
•
Nicholas F. Dummer
•
•
David J. Willock
Stuart H. Taylor
•
•
•
David W. Knight
•
Received: 10 May 2012 / Accepted: 4 July 2012 / Published online: 19 July 2012
Ó Springer Science+Business Media, LLC 2012
Abstract The oxidative esterification of a homologous
series of diols (1,3-propanediol,2-methyl-propanediol and
required, as well as the presence of halide anions, make this
reaction environmentally unfavourable. Several relatively
facile aldehyde esterifications with alcohols have been
reported, in which stoichiometric amounts of oxygen
donors are utilised, e.g. KHSO5 or MnO2 [5]. These
methods require several reaction steps and produce
unwanted by-products. Therefore, development of a cata-
lytic, single-step, direct oxidative methyl esterification of
alcohols under mild conditions is highly desirable from
both economic and environmental points of view. Solid
catalysts based on supported noble-metal nanoparticles
have attracted tremendous recent attention owing to their
unique catalytic properties for a broad spectrum of organic
transformations, especially for the aerobic oxidation of
alcohols. The formation of methyl esters by oxidising a
primary alcohol over supported gold nano-particle catalysts
in methanol has been reported [6–8]. The first examples of
oxidative esterification of diols were reported by Kunugi
[9, 10]. More recently, Hayashi et al. [6] showed that the
selective oxidative esterification of ethylene glycol to
methyl esters, by means of molecular oxygen, could occur
using nano-gold particles supported on metal oxides as a
slurry. Christensen and co-workers [11] have reported
2
,2-dimethyl-1,3-propanediol) with methanol has been
investigated using titania-supported gold, palladium and
gold–palladium catalysts using molecular oxygen. The gold–
palladium catalysts showed the highest activity and 1,3-pro-
panediol was the most reactive while the additional methyl
groups decreased the reactivity. However, it is possible to
achieve high selectivity to methyl 3-hydroxypropionate and
2
-methyl-3-hydroxyisobutyrate by mono-oxidations.
Keywords Gold catalysis Á Gold palladium alloy
nanoparticles Á Oxidative esterification Á Diol oxidation
1
Introduction
Methyl esterification is a ubiquitous methodology in
organic synthesis [1]. Methyl esters are very useful
chemical intermediates with respect to atom economy and
versatility for further transformations [2]. Synthesis of
esters from alcohols is traditionally a two-step procedure
that includes conversion to carboxylic acids or activated
carboxylic acid derivatives such as acid anhydrides or acid
chlorides [3]. An alternative approach involves the car-
bonylation of aryl halides by transition-metal catalysts [4].
However, the high temperature and high CO pressures
similar work with 1 % Au/TiO as catalyst and reported
2
90 % selectivity to methyl 3-hydroxypropionate at 94 %
conversion of 1,3-propanediol. Catalytic oxidation of
2-methyl-1,3-propanediol with Pd-Bi/Al O in the pres-
2
3
ence of MeOH and air was reported by Tagawa et al. [12]
to give 53 % methyl 3-hydroxyisobutyrate. The oxidation
of 2,2-dimethyl-1,3-propanediol has, to date, received little
attention, Hong et al. [13] showed that using Pd/C as cat-
alyst in air led to the formation of 3-hydroxy-2,2-dimeth-
ylpropionic acid, while a gas-phase oxidation of this diol
over alkali-exchanged zeolites gives 2-methylpropanal,
2-methylbutanal and traces of 3,3-dimethyloxetane [14].
T. Kotionova Á C. Lee Á P. J. Miedziak Á
N. F. Dummer Á D. J. Willock Á A. F. Carley Á
D. J. Morgan Á D. W. Knight Á S. H. Taylor Á
G. J. Hutchings (&)
Cardiff Catalysis Institute, School of Chemistry, Cardiff
University, Main Building, Park Place, Cardiff CF10 3AT, UK
e-mail: hutch@cf.ac.uk
1
23

Oxidative Esterification of Homologous 1,3-Propanediols
1115
We have previously investigated the oxidative esterifi-
cation of 1,2-propanediol to form methyl lactate and
methyl pyruvate [15] using supported Au, Pd and AuPd
nanoparticles. In this paper we have now extended this
160 eV (for survey scans) or 40 eV (for detailed scans).
Samples were mounted using double-sided adhesive tape
and binding energies referenced to the C(1 s) binding
energy of adventitious carbon contamination which was
taken to be 284.7 eV.
study, we have investigated if homologous C -based
3
propanediols are able to undergo selective oxidation to
their corresponding methyl hydroxy-esters by carrying out
a noble-metal catalysed aerobic oxidation in methanol. The
substrates investigated were 1,3-propanediol,2-methyl-1,3-
propanediol and 2,2-dimethyl-1,3-propanediol. The choice
of these substrates was made in order to have polyols with
hydroxyl groups in primary positions with additional
methyl groups. In this paper we report the findings of this
investigation.
2.3 Oxidative Esterification Reactions
Oxidative esterification reactions were carried out using a
50 ml Radleys low pressure reactor with a maximum
working pressure of 4 bar. The reactor was charged with
1,3-propanediol,2-methyl-1,3-propanediol or 2,2-dimethyl-
1,3-propanediol (1.5 mmol), sodium hydroxide (1.5 mmol,
0.06 g), methanol (7.9 g) and catalyst (substrate/metal
molar ratio = 500/1). The reaction vessel was sealed,
purged 3 times with oxygen and pressurized to 3 bar with
2
Experimental
O ; the oxygen inlet remained open throughout the reaction
2
so that oxygen was replenished as it was consumed. The
reaction mixture was stirred (1,000 rpm) and heated to
80 °C. Reactions were cooled to 20 °C periodically and
samples were taken. The reactor was then recharged with
oxygen and reheated to 80 °C. The isolated reaction mix-
2
.1 Preparation of the Catalysts by the
Sol-Immobilisation Method
An aqueous solution of PdCl2 (Johnson Matthey) and
1
HAuCl Á3H O of the desired concentration was prepared.
tures were concentrated under vacuum and analysed by H
4
2
1
3
Polyvinylalcohol (PVA) (1 wt% solution, Aldrich,
and C NMR spectroscopy. Conversions and selectivities
were calculated from NMR spectra via integration of the
starting material and the products. These analyses were
confirmed by additional GC analysis which was carried out
using a Varian 3,800 chromatograph equipped with a CP
8,400 autosampler and CP-wax 52 column. Products were
identified by comparison with authentic samples, and
quantification, and hence yields, were established using an
external standard.
MW = 10,000, 80 % hydrolyzed) was added (PVA/Metal
(
by wt) = 1.2); 0.1 M freshly prepared solution of NaBH4
([96 %, Aldrich, NaBH /Au (mol/mol) = 5) was then
4
added to form a dark-brown sol. After 30 min of sol gen-
eration, the colloid was immobilized by adding titania
2
oxide (Degussa P25, SA = 49 m /g, 80 % anatase), acid-
ified to pH 1 with sulfuric acid, under vigorous stirring.
The amount of support material required was calculated so
as to have a total final metal loading of 1 wt%. After 2 h
the slurry was filtered, the catalyst washed thoroughly with
distilled water (neutral mother liquors) and dried at 120 °C
overnight. The catalyst was prepared using a 1:1 molar
2.4 Results and Discussion
The three catalysts used in this study 1 %Au/TiO , 1 %Pd/
2
ratio of Au:Pd (denoted as 1 %AuPd(1:1)/TiO ). Mono-
2
TiO2 and 1 %AuPd(1:1)/TiO2 have been extensively
characterised previously using scanning transmission
electron microscopy [16–19]. The metal particle size for
the catalysts prepared by sol-immobilisation was observed
to be between 4 and 7 nm [16, 17]. The AuPd nanoparticles
were all found to comprise homogeneous alloys and no
core–shell structures were observed. As the catalysts had
been well characterised previously we have used them in
the present study. However, we have carried out some
additional XPS characterisation (Table 1) to complement
the detailed microscopy studies [16–19]. The XPS data
showed that the catalysts contain predominantly metallic
metallic catalysts containing gold or palladium (for the Pd
monometallic catalysts a small amount of aqueous HCl was
added to aid dissolution of PdCl ) were also prepared using
2
similar methodology, and these are denoted 1 %Au/TiO2
and 1 %Pd/TiO . Analysis confirmed the metal concen-
2
tration was as specified, and we have used these catalysts
for redox reactions previously and have found them to be
reusable without significant loss of activity or metal
content.
2
.2 Catalyst Characterisation
0
0
Au , and Pd species as indicated by binding energies of
83.1 and 334.9 eV for the respective metals; the low
binding energy of the Au(4f7/2) signal attributable to small
(typically \5 nm) particles as observed by Radnik et al.
The catalysts were characterised using X-ray photoelectron
spectroscopy (XPS). Spectra were recorded on a Kratos
Axis Ultra DLD spectrometer employing a monochromatic
AlKa X-ray source (120 W) and analyser pass energies of
[20] for Au on TiO prepared by several methods. The
2
123

1
116
T. Kotionova et al.
Table 1 XPS-derived surface elemental compositions for the cata-
lysts prepared by sol-immobilisation
with notable amounts of the sodium salt of 3-hydroxyprop-
ionic acid. It appears that methyl 3-hydroxypropionate is
stable under these conditions and not oxidised further to other
products (only traces of dimethyl malonate were detected).
The 3-hydroxypropionic acid may arise from the hydrolysis
of methyl 3-hydroxypropanoate formed during the reaction
(Scheme 1). Formation of the corresponding ether, methyl
3-methoxypropanoate, was unexpected but its formation can
Catalyst
Au (at%)
Fresh Used Fresh Used
0.30 0.12 Pd ? Pd
Pd (at%)
Oxidation state
0
2?
1
1
1
%Pd/TiO
%Au/TiO
%AuPd/TiO
2
-
-
0
2
0.26
0.29
0.18
0.31
-
-
Au
0
0.80 Au , Pd
0
2
0.12
easilybe rationalizedbya base-catalysedE cB elimination of
1
Data are shown for the unused catalysts and also after use for
,3-propanediol oxidation
water from methyl 3-hydroxypropanoate to give methyl
acrylate which would be expected to undergo very rapid
Michael addition of methanol under such conditions
(Scheme 1). Direct Williamson-type O-alkylation seem
much less likely given the reagents present. Formic acid and
methyl formate are also observed and these originate from
oxidation of methanol that proceeds in parallel with the oxi-
dative esterification. Low amounts of carbon oxides, partic-
1
monometallic Pd/TiO catalyst does exhibit some Pd(II) as
2
evidenced by a shoulder to higher binding energy
(
336.7 eV); however, after reaction there is no discernible
amount of Pd(II). For the monometallic catalysts the
decrease in metal content in the external layer of the cat-
alyst after the reaction could reflect sintering of the metal
particles. The redistribution of the elements on the surface
would also lead to similar phenomenon. However, the
ularly CO , are observed as theproducts of the over-oxidation
2
of the diol or methanol.
The monometallic Pd catalyst supported on titania was
more active than the Au catalysts (Table 2). However, the
monometallic Pd catalysts only gave ca. 80 % carbon mass
balance for the identified products. This was because the
Au–Pd/TiO sample reveals an increase in the amount of
2
Pd, whilst the Au remains constant (within experimental
accuracy) and may be attributed to highly re-dispersed Pd
sites. The observed molar ratio of metals for the bimetallic
catalysts is consistent with the absence of core–shell
structures.
monometallic Pd catalysts gave CO as a product but this
2
could not be collected and accurately quantified in this
reactor set up. Previous studies of alcohol oxidation with
monometallic Pd catalysts have also shown this to be the
case [21]. By comparison, the bimetallic 1 %Au–Pd/TiO2
catalyst was moderately active and retained high selectivity
Oxidative esterification of 1,3-propanediol was carried out
over 1 %Au/TiO , 1 %Pd/TiO and 1 %AuPd(1:1)/TiO .
2
2
2
The main product observed was methyl 3-hydroxypropionate
Scheme 1 A proposed reaction
scheme of oxidation of 1,3-
propanediol in methanol in
presence of base over noble-
metal catalysts. Observed
products are highlighted in
boxes
1
23

Oxidative Esterification of Homologous 1,3-Propanediols
1117
Table 2 1,3-propanediol oxidative esterification
Table 4 Oxidation of 2,2-dimethyl-1,3-propanediol
Catalyst Time Conversion MHP HPA
Catalyst
Time Conversion 3MHP 3HPA
(%) selectivity (%) selectivity (%)
HPAld
selectivity selectivity selectivity
(h)
(h)
(%)
(
%)
(%)
(%)
1
1
1
%Au/TiO
2
2
4
10
18
39
41
61
69
20
28
45
87
84
82
95
88
82
88
82
80
12
15
17
5
1
1
%Au/
TiO
2
4
7
10
30
11
12
15
18
22
25
60
60
73
15
21
22
61
62
60
40
40
26
15
17
16
11
28
32
–
–
2
2
2
2
4
2
4
4
2
4
4
24
2
4
23
2
4
23
–
%Pd/TiO
2
%Pd/
TiO
69
62
61
26
10
7
12
7
2
%AuPd
1:1)/TiO
11
18
19
(
2
1 %AuPd
1:1)/
TiO
(
2
3MHP methyl 3-hydroxypropionate, 3HPA 3-hydroxypropanoic acid
sodium salt)
(
MHP methyl 3-hydroxypivalate, HPA 3-hydroxypivalic acid, HPAld
-hydroxypivalic aldehyde
3
Table 3 Oxidation of 2-methyl-1,3-propanediol
Catalyst Time Conversion MHB
group is influenced by the presence of a methyl ester group
making it more difficult to oxidise (Scheme 2). Oxidative
esterification of 2-methyl-1,3-propanediol leads to the
formation of methyl 3-hydroxyisobutyrate in high selec-
tivities (Table 3; Scheme 2). Reactions over 1 %Pd/TiO2
resulted in a selectivity of 99 at 21 % conversion after 2 h;
however, the selectivity decreased at longer reaction times.
HBA
selectivity (%) selectivity (%)
(h)
(%)
1
1
1
%Au/TiO
2
2
4
10
12
46
21
30
62
16
22
37
60
75
93
99
72
55
90
91
79
39
14
7
2
2
2
4
2
4
4
2
4
4
%Pd/TiO
2
–
Conversely reactions over 1 %Au/TiO retained selectivity
2
28
45
10
9
to the major product at higher conversion (93 % selectivity
to methyl hydroxyisobutyrate at 46 % conversion after
%AuPd
1:1)/TiO
2
24 h). This shows the influence that Au has on modifying
(
the catalytic performance of Pd by maintaining the selec-
tivity to the desired ester product. From this point of view,
this process can be considered as a potential route from
20
MHB methyl hydroxyisobutyrate, HBA hydroxyisobutyric acid
2-methyl-propanediol through methyl 3-hydroxyisobuty-
to the major product (80 % selectivity at 45 % conversion
after 24 h) and in this case the mass balance was closed.
rate to a large scale chemical such as methyl methacrylate
[22]. These results are essentially the same as those
obtained from oxidations of the parent 1,3-propanediol
(Table 2).
Nevertheless, the performance of the 1 %Pd/TiO catalyst
2
showed that it is active after 2 h giving 90 % selectivity to
the major product at 41 % conversion, which is competi-
tive with the results achieved in previous studies [6, 11].
Oxidations of 2-methyl-1,3-propanediol and 2,2-dime-
thyl-1,3-propanediol were carried out under the same
conditions as the experiments with 1,3-propanediol. The
oxidation results showed hydroxy-ester and hydroxy-acid
to be the major reaction products (Tables 3, 4). Interest-
ingly, the oxidation is taking place at only one alcohol
functionality in all of the three substrates studied. This
chemoselectivity may indicate that the second alcohol
The peculiarity of 2,2-dimethyl-1,3-propanediol oxida-
tion was that the hydroxy-aldehyde was obtained as the
reaction product (Scheme 3), in the reactions catalysed by
palladium (Table 4). When oxidizing 1,3-propanediol and
2-methyl-1,3-propanediol in methanol, we observed no
aldehyde formation, suggesting that it is an intermediate
that is rapidly oxidised either into the corresponding
hydroxy-acid or hydroxy-ester. The reactivity of 2,2-
dimethy-1,3-propanediol is higher over 1 %Au–Pd(1:1)/
TiO , although oxidation does not continue to completion
2
Scheme 2 A proposed reaction
scheme of oxidation of
2-methyl-1,3-propanediol in
methanol in presence of base
over noble-metal catalysts
123

1
118
T. Kotionova et al.
Scheme 3 A proposed reaction scheme of oxidation of 2,2-dimethyl-1,3-propanediol in methanol in presence of base over noble-metal catalysts
and hence we observe a decrease in the rate of reaction
with this system over longer reaction times. By compari-
50
40
30
20
10
0
son, prolonged reactions with 1 %Au/TiO indicated that
2
activity is retained (Fig. 1), and appears to maintain
selectivity to the major product.
Methyl substituted homologous propanediols are oxi-
dised more slowly than 1,3-propanediol. With respect to
the general trend of reactivity of the different propanediols
substrates to oxidation, the rate of oxidation decreases
with the addition of methyl groups, following the order:
1,3-propanediol[2-methyl-1,3-propanediol [2,2-dimethyl-
1
,3-propanediol. Only in the case of 1 %Au/TiO is the
2
initial reactivity of all three substrates found to be similar
(
Figs. 2, 3, 4).
The difference in reactivity between 1,3-propanediol
and its homologs is likely due to the different steric hin-
drance around the OH group in the different alcohols. In
particular molecular modeling using B3LYP/6-31G(d,p)
0
5
10
15
20
25
30
Time / h
(
[
All calculations carried out using the Gaussian09 package
23]) of the first dehydrogenation product, 3-hydroxy-
Fig. 2 Oxidation of homologous propanediols using 1 %Au/TiO2.
Conversion of 1,3-propanediol (filled square), conversion of
2-methyl-1,3-propanediol (filled diamond), conversion of 2,2-
dimethyl-1,3-propanediol (filled triangle)
propanal and its methylated analogues, show that the
lowest energy structures have the aldehyde group away
8
0
50
45
40
35
30
25
20
15
10
5
0
100
90
80
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
0
10
20
30
0
5
10
15
20
25
30
Time / h
Time / h
Fig. 1 Effect of time on the oxidative esterification of 2,2-dimethyl-
,3-propanediol over 1 %Au/TiO . Conversion of 2,2-dimethyl-1,3-
propanediol (filled square), selectivity to methyl 3-hydroxypivalate
filled diamond), selectivity to 3-hydroxypivalic acid (filled triangle)
Fig. 3 Effect of time on the oxidation of homologous propanediols
over 1 %Pd/TiO . Conversion of 1,3-propanediol (filled square),
conversion of 2-methyl-1,3-propanediol (filled diamond), conversion
of 2,2-dimethyl-1,3-propanediol (filled triangle)
1
2
2
(
1
23

Oxidative Esterification of Homologous 1,3-Propanediols
1119
50
40
30
20
10
0
Table 5 Calculated (B3LYP/6-31G(d,p)) [23] relative energies for
conformations of 1,3-propanediol oxidative esterification
Molecule
Conformer
[a]
Relative
energy/kJ/
mol
1
2
2
3
2
,3-Propanediol
i
0
16
0
ii
i
-Methyl-1,3-propanediol
,2-Dimethyl-1,3-propanediol
-Hydroxypropanaldehyde
-Methyl-3-hydroxypropanaldehyde
ii
i
19
0
ii
i
19
0
ii
i
19
0
ii
i
14
0
0
5
10
15
20
25
30
2,2-Dimethyl-3-
hydroxypropanaldehyde
Time / h
ii
16
Fig. 4 Oxidation of homologous propanediols using 1 %AuPd(1:1)/
TiO . Conversion of 1,3-propanediol (filled square), conversion of
-methyl-1,3-propanediol (filled diamond), conversion of 2,2-
Note [a] Conformers defined as in Fig. 5
2
2
centre has to be approached by methanol to form the hemi-
acetal (1-methoxy-1,3-propanediol) shown in Scheme 1
and so this step in the reaction will be slower for methyl-
ated substrates. We suggest that since the double bond of
the aldehyde group will interact more strongly with the
metal component of the catalyst than it will with the
alcohol of the substrate; site blocking by this intermediate
dimethyl-1,3-propanediol (filled triangle)
from substituents at the 2-position (Table 5). This confor-
mation is stabilized by an internal hydrogen bond. For the
substrates with a methyl group in the 2-position this results
in steric crowding of the aldehyde C atom (Fig. 5). This
Fig. 5 a and b Chemical structure representations of the i and ii
conformations for the examples of 1,3-propanediol and hydroxypro-
panaldehyde, in each case an internal hydrogen bond stabilises
dimethylhydroxypropanaldehyde and 2,2-dimethyl- hydroxypropan-
aldehyde in the i conformation. CPK spheres are set at 0.7 of the van
der Waals radius for each atom. Atom colours, C : grey, O : red and H
: white
the
i conformation, c and d optimised structures for 2,2-
123

1
120
T. Kotionova et al.
will occur. This will be particularly true for catalysts
containing Pd, since the aldehyde interaction with Au
particles will be considerably weaker than with Pd. A more
detailed study is still underway, at present we suggest that
the substrate reactivity is also affected by diffusion of
reactants within the catalyst and the strength of surface
interaction of reactants with the catalyst surface. Both of
these effects will be less important for 1,3-propanediol
where the OH groups are more accessible than for
ideas for the benefit of business—increasing sustainable economic
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Herzing AA, Watanabe M, Kiely CJ, Knight DW, Hutchings GJ
1
1
1
1
1
1
1
1
3
Conclusions
The oxidation of 1,3-propanediol,2-methyl-propanediol
and 2,2-dimethyl-1,3-propanediol over supported noble-
metal catalysts in methanol was investigated. Using sup-
ported gold palladium catalysts it was possible to tune the
catalytic reactivity and selectivity to the desired products
for the oxidative esterification of 1,3-propanediols to
hydroxy-esters in the liquid-phase using molecular oxygen.
Introducing the methyl substituents into the propanediol
substrate decreased the reactivity of the substrate to oxi-
dation. It was possible to achieve high selectivity to methyl
1
2
2
(
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3
-hydroxy-propionate and 2-methyl-3-hydroxyisobutyrate
which potentially could be applied to the production of
chemicals such as methyl acrylate and methyl methacrylate
on a large scale.
Acknowledgments This work formed part of the Glycerol Chal-
lenge and Tennants Fine Chemicals Ltd and the Technology Strategy
Board are thanked for their financial support. This project is co-fun-
ded by the Technology Strategy Boards Collaborative Research and
Development programme, following an open competition. The
Technology Strategy Board is an executive body established by the
Government to drive innovation. It promotes and invests in research,
development and the exploitation of science, technology and new
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