Catalytic performance of divanadium-substituted molybdophosphate acid, H5PMo10V2O40, in liquid-phase esterification of hexanoic acid

Article abstract of DOI:10.1080/10426500802625784

Divanadium-substituted molybdophosphate acid, H₅PMo ₁₀V₂O₄₀, efficiently catalyzes the synthesis of ethylhexanoate from hexanoic acid and ethanol at reflux temperature and under solventless conditions. Comparison of H₅PMo₁₀V ₂O₄₀ with the other heteropolyacids showed that this catalyst gives the highest total yield of ethylhexanoate. The effects of the molar ratio of acid:alcohol, reaction times, and temperatures were studied.

Full text of DOI:10.1080/10426500802625784

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Catalytic Performance of
Divanadium-Substituted
Molybdophosphate Acid,
H PMo V O , in Liquid-Phase
5
10 2 40
Esterification of Hexanoic Acid
a
b
Fatemeh F. Bamoharram , Majid M. Heravi , Mina
a
a
Roshani & Hamzeh Mir
a
Department of Chemistry , Islamic Azad
University–Mashhad Branch , Mashhad, Iran
b
Department of Chemistry, School of Sciences ,
Azzahra University , Vanak, Tehran, Iran
Published online: 18 Nov 2009.
To cite this article: Fatemeh F. Bamoharram , Majid M. Heravi , Mina Roshani
&
Hamzeh Mir (2009) Catalytic Performance of Divanadium-Substituted
Molybdophosphate Acid, H PMo V O , in Liquid-Phase Esterification of Hexanoic
5
10 2 40
Acid, Phosphorus, Sulfur, and Silicon and the Related Elements, 184:12, 3119-3125,
DOI: 10.1080/10426500802625784
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Phosphorus, Sulfur, and Silicon, 184:3119–3125, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802625784
Catalytic Performance of Divanadium-Substituted
Molybdophosphate Acid, H PMo V O , in Liquid-Phase
5
10
2
40
Esterification of Hexanoic Acid
1
2
Fatemeh F. Bamoharram, Majid M. Heravi,
1
1
Mina Roshani, and Hamzeh Mir
Department of Chemistry, Islamic Azad University–Mashhad Branch,
Mashhad, Iran
1
2
Department of Chemistry, School of Sciences, Azzahra University,
Vanak, Tehran, Iran
Divanadium-substituted molybdophosphate acid, H5PMo10V2O40, efficiently cat-
alyzes the synthesis of ethylhexanoate from hexanoic acid and ethanol at reflux
temperature and under solventless conditions. Comparison of H5PMo10V2O40 with
the other heteropolyacids showed that this catalyst gives the highest total yield of
ethylhexanoate. The effects of the molar ratio of acid:alcohol, reaction times, and
temperatures were studied.
Keywords Catalyst; esterification; ethylhexanoate; heteropolyacid; hexanoic acid;
vanadium (V)-substituted heteropolymolybdates
INTRODUCTION
Esterification, with no doubt, continues to play important roles both in
organic synthesis and in the chemical industry. Since acid catalysis is
one of the most popular methods for esterification, numerous articles
are available that relate to this purpose.1–3 When the substrates are
acid-resistant, the reaction is usually carried out in the presence of
Brønsted acids such as HCl, HBr, H2SO4, NaHSO4, ClSO3H, NH2SO3H,
H3PO4, HBF4, AcOH, and camphorsulfonic acid. Most of these catalysts
are corrosive and virulent, and need to be neutralized at the end of
reaction with various bases.
All chemical processes, not only esterification, should be performed
as “green” as possible.4 For this reason, various solid acids are utilized
,5
Received 8 July 2008; accepted 10 November 2008.
Address correspondence to Fatemeh F. Bamoharram, Department of Chemistry, Is-
lamic Azad University–Mashhad Branch, Mashhad, Iran or Majid M. Heravi, Depart-
ment of Chemistry, School of Sciences, Azzahra University, Vanak, Tehran, Iran. E-mail:
fbamoharram@mshdiau.ac.ir
3119

3120
F. F. Bamoharram et al.
6
for esterification reactions. Metal-containing Lewis acid catalysts,
7
8
Nafion-H, amberlyst 15, amberlyst IR120, wolfatit KSP200 (an
ion-exchange resin), zeolites,10 Nb2O5, FeCl3 supported on sali-
9
11
1
2
13
14
cylic resin, Fe(ClO4)3 (ROH)6/SiO2, NaHSO4/SiO2, phosphorus ox-
1
5
16
17
18
ides, Ph3SbO/P4S10,1 ZrO2 and Mo-ZrO2, graphite bisulfate, nat-
9
20
21
ural montmorillonite, niobic acid, sulfated oxides, and heteropoly-
2
2,23
acids,
were reported in the literature for esterification reactions.
Nowadays, the research and development in the field of esterifica-
tion has been directed towards the use of catalysts with strong Brønsted
acidity, as they have enhanced performance, are inexpensive, less haz-
ardous to the environment, easily prepared being recyclable and sta-
2
4–26
ble.
Recently, heteropolyacids have attracted much attention and
2
7–29
being studied in many esterification reactions.
However, because their special structural topology and versatile
properties, which can be tuned at the atomic/molecular levels through
modification of the structure type, the central heteroatom, and the
transition metal-substituted atoms,3
0–33
there is still a broad scope for
research on various heteropolyacids in esterification.
The aim of the present study, in continuation of our earlier work
3
4,35
on catalytic applications of heteropolyacids,
is to assess and use
the heteropolyacid catalysts in the synthesis of ethylhexanoate. The
selected catalyst was vanadium (V)-substituted heteropolymolybdate,
H5PMo10V2O40.
We focused our study on the relationship between the yields of ester
produced with molar ratio of acid:alcohol, alcohol:acid, temperatures,
and reaction conditions.
RESULTS AND DISCUSSION
The esterification of hexanoic acid with ethanol is an acyl nucleophilic
substitution. The reaction is relatively slow and needs activation either
by high temperature or by a catalyst. This work has been performed to
study the catalytic behavior of H5PMo10V2O40 in esterification of hex-
anoic acid with ethanol and was also extended to the other heteropoly-
acids such as H6PMo9V3O40, H4PMo11VO40, H4SiW12O40, H3PMo10O40,
and H6P2W18O62.
The results are shown in Table I. By comparison, it is evident that
H5PMo10V2O40, is the catalyst of choice; all other catalysts were found
to be less active and to have nearly identical activity. A significant in-
terpretation for different activities of heteropolyacids is very difficult.
Acidity, basicity, and pseudo liquid behavior are the principal factors

Divanadium-Substituted Molybdophosphate
3121
TABLE I Effect of Various Catalysts on Esterification of
Hexanoic Acid in the Presence of Ethanol
Entry
Catalyst
Yield of ethylhexanoate (%)a
1
2
3
4
5
6
H5PMo10V2O40
H4PMo11VO40
H6PMo9V3O40
H3PMo12O40
H4 SiW12O40
H6P2W18O62
93.8
10.9
11.8
11.8
11.9
11.9
aYields determined by GC/MS.
governing the acid catalysis of solid heteropolyacids. The acidic prop-
erties are mainly controlled by (i) the structure and composition of the
heteropolyanion itself, (ii) the counter cations, and (iii) the dispersion
on supports. The acid strength can be controlled mainly by (i), and the
number of acid sites is greatly influenced by (ii) and (iii). In addition,
the soft basicity of the heteropolyanion itself sometimes plays an im-
portant role for high catalytic activity in acid-catalyzed reactions. The
acid strength for hydrogen forms in the solid state reflects in general
the acidity in solution, and it decreases when W is replaced by Mo and
when the central P atom is replaced by Si for Keggin heteropolyacids,
3
6
which are stronger acids than Dawson heteropolyacids. However, it
was reported for heteropolyacids of Mo that its acid strength in solution
is not correlated with that in solid.37 Additionally, the acid properties
of heteropolyacids are quite well documented in terms of dissociation
constants and Hammett acidity function.3
8,39
As can be seen in Table I, among the studied catalysts,
H5PMo10V2O40 gives the highest yield in this reaction. Despite the
importance and versatility of reactions catalyzed by vanadium (V)-
substituted heteropolymolybdates (HPA-n), there is only limited infor-
mation about their acidic and redox mechanisms. HPA-n in solution is
an extremely complex system, and the higher the n value, the more
complex the system is. The HPA-n solutions contain a great number of
4
0,41
polyanions, positional isomers, and monomeric metallospecies.
One of the important factors that affects the activity of polyanions is
the energy gap between the highest occupied molecular orbital (HOMO)
and the lowest unoccupied orbital (LUMO). We believe that the energy
and composition of the LUMOs have significant effects on the activity
of the studied heteropolyacids. In Keggin series, for H5PMo10V2O40,
the metal substitution may modify the energy and composition of the
LUMO. In addition, the higher activity of the H5PMo10V2O40 can be

3122
F. F. Bamoharram et al.
TABLE II Effect of Molar Ratio of Reactants on the Yield of Produced
Ethylhexanoate in the Presence of H
5
2 40
PMo10V O
Molar ratio of
acid:alcohol
Yield of
Molar ratio of
alcohol:acid
Yield of
ethylhexanoate (%)
ethylhexanoate (%)a
a
Entry
1
2
3
4
5
6
7
1:1
1:2
1:3
1:4
1:5
1:6
1:7
88.2
37.5
56.0
40.2
23.8
25.3
21.3
1:1
1:2
1:3
1:4
1:5
1:6
1:7
18.7
31.5
16.2
16.1
14.8
13.3
93.8
aYields determined by GC/MS.
attributed to the concentration of protons, strong acid sites that require
+
in esterification reactions and produced VO2 in reaction course as an
active species.
With the catalyst of choice for this reaction, H5PMo10V2O40, the reac-
tion was carried out with various molar ratios of acid:alcohol. Different
molar ratios of acid:alcohol from 1:1 to 1:7 were reacted at reflux tem-
perature for 6 h. The results are presented in Table II. As can be seen,
a nonlinear trend in yields is observed with increased molar ratios of
alcohol:acid from 1:1 to 7:1. The yield of ester increased for 1:1 ratio
and followed by a decrease thereafter. Although the increase in alco-
hol content might be expected to increase conversion, there might be
dilution of hexanoic acid; hence the expected increase in conversion
must be balanced by decrease in the conversion due to the increase in
dilution of hexanoic acid. There might be dilution of acid by increased
alcohol contents, thereby preventing hexanoic acid adsorption on the
Brønsted acid sites. The reaction was also studied by using more acid
content (Table I).
When the molar ratio of acid:alcohol increased from 1:1 to 7:1, the
yield of ethylhexanoate increased. Since hexanoic acid probably under-
goes chemoadsorption on the Brønsted acid sites, an increase in acid
content increased the yield.
In order to verify the effectiveness of the catalyst, we also refluxed
the reaction mixture in the absence of the catalyst for 6 h. After 6 h,
the reaction mixture was analyzed by GC/MS. The yield was found to
be only 4–5%.
The effect of reaction time on the yield of ethylhexanoate also was
studied using H5PMo10V2O40 at reflux temperature with a molar ratio
of 1:7 for alcohol:acid. Yields increased from 16% at 2 h to a maximum

Divanadium-Substituted Molybdophosphate
3123
of 93.9% at 6 h. An increase in the yield was noticed with increase in the
duration of the reaction time. This observation is an expected feature
for this kind of reaction.
The reaction was also carried out at room temperature with a molar
ratio of 1:7 for alcohol:acid for 6 h. In this case, the progress of reaction
was very slow and the yield was negligible.
This result shows that the reaction needs more energy to proceed.
This energy may be required to reduce intermolecular associations of
alcohol and to avoid clustering of alcohol around the Brønsted acid sites
by hydrogen bonding.
In summary, the results show potential use of the heteropolyacid
H5PMo10V2O40 as a catalyst in the esterification reaction. We have
shown in this study that the reaction is sensitive to the choice of catalyst
and the amount of acid and alcohol. The reaction provides a simple and
highly efficient catalytic method for synthesis of aliphatic esters under
mild conditions.
EXPERIMENTAL
The reaction mixture was analyzed by GC/MS (GC-Mass model: 5973
network mass selective detector, GC 6890 Agilent). The products were
also characterized by comparison of their spectroscopic data with those
of authentic samples. Yields were determined by GC/MS.
All chemicals were purchased from Merck Company and were used
without further purification.
The catalyst, H5PMo10V2O40, was synthesized by acidification-
4
2
etherate from sodium molybdate, sodium vanadate, and sulfuric acid.
Other catalysts were obtained from commercial sources.
Typical Procedure
In a typical reaction, the catalytic esterification of hexanoic acid was
carried out in a round bottom flask equipped with a magnetic stir-
rer, thermometer, and a reflux condenser. Catalyst (0.0001 mol) was
added to a mixture of hexanoic acid (0.05 mol) and alcohol (0.0075
mol). The reaction mixture was stirred and refluxed for 6 h. Different
reaction runs were conducted by varying the reaction parameters such
as molar ratio of reactants, the catalyst type, temperature, and reaction
time.

3124
F. F. Bamoharram et al.
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