Esterification of acetic acid with n-butanol using molybdenum oxides supported on γ-alumina

Article abstract of DOI:10.1007/s10562-010-0431-z

Two γ-alumina-supported molybdenum oxide catalysts with different Mo loadings were prepared by the impregnation of a precipitated alumina. The alumina support and the supported catalysts were characterized using X-ray diffraction, N₂ adsorption and NH₃-TPD techniques and tested in the liquid-phase esterification of acetic acid with n-butanol. The effects of the reaction time, of the molar ratio of the reactants, of the speed of agitation and of the mass fraction of the catalyst on the catalytic properties were studied. After only 120 min of reaction at 100 °C with an n-butanol to acetic acid mole ratio equal to 3 the conversion reached 81% in the presence of the supported catalyst containing 10 wt% MoO₃ (10Mo-Al₂O₃ sample). The conversion as well as the total NH₃-TPD acidity increased in the following order: Al ₂O₃ < 5Mo-Al₂O₃ (5 wt% MoO ₃/Al₂O₃) < 10Mo-Al₂O₃. The selectivity of n-butyl acetate was, in all cases, 100%. No loss in catalytic activity and product selectivity after three cycles with 10Mo-Al ₂O₃ catalyst was observed. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-010-0431-z

Catal Lett (2010) 140:32–37
DOI 10.1007/s10562-010-0431-z
Esterification of Acetic Acid with n-Butanol Using Molybdenum
Oxides Supported on c-Alumina
´
´
•
•
•
´
´
Gheorghita Mitran Eva Mako Akos Redey
Ioan-Cezar Marcu
Received: 21 May 2010 / Accepted: 11 August 2010 / Published online: 26 August 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Two c-alumina-supported molybdenum oxide
catalysts with different Mo loadings were prepared by the
impregnation of a precipitated alumina. The alumina sup-
port and the supported catalysts were characterized using
X-ray diffraction, N2 adsorption and NH3-TPD techniques
and tested in the liquid-phase esterification of acetic acid
with n-butanol. The effects of the reaction time, of the
molar ratio of the reactants, of the speed of agitation and of
the mass fraction of the catalyst on the catalytic properties
were studied. After only 120 min of reaction at 100 °C
with an n-butanol to acetic acid mole ratio equal to 3 the
conversion reached 81% in the presence of the supported
catalyst containing 10 wt% MoO3 (10Mo–Al2O3 sample).
The conversion as well as the total NH3-TPD acidity
increased in the following order: Al2O3 \ 5Mo-Al2O3
(5 wt% MoO3/Al2O3) \ 10Mo–Al2O3. The selectivity of
n-butyl acetate was, in all cases, 100%. No loss in catalytic
activity and product selectivity after three cycles with
10Mo–Al2O3 catalyst was observed.
Keywords Esterification Á Acetic acid Á n-Butanol Á Butyl
acetate Á c-Alumina-supported molybdenum oxide
1 Introduction
Esters are most commonly prepared by direct esterification
of carboxylic acids with alcohols in the presence of acid
catalysts [1]. Both homogeneous and heterogeneous
catalysts have been used to accelerate the esterification
reactions. While the mineral acids, such as H2SO4, HCl,
HF, ClSO2OH, can be given as examples of the homogenous
catalysts [2], cation-exchange resins [3, 4], zeolites [5, 6]
or supported heteropolyacids [2, 7–9] could serve as het-
erogeneous catalysts.
Even though the homogeneous acid catalysts are widely
used at industrial scale for their low price, they are toxic,
corrosive and often hard to be recovered from the reaction
solution. On the other hand, the solid catalysts have some
advantages: they can be separated from liquid reaction
mixture by filtration or decantation, have high selectivity
and, last but not least, they are environmentally friendly.
Therefore, the replacement of the homogeneous acid cat-
alysts by solid acid catalysts is a challenge of heteroge-
neous catalysis. To our knowledge, there are no studies
investigating alumina-supported molybdenum oxides as
catalysts for the esterification of n-butanol with acetic acid.
Butyl acetate, as the most of alkyl acetates, is generally
used as solvent, as component in flavoring and as additive
in perfumes [1].
G. Mitran Á I.-C. Marcu (&)
Department of Chemical Technology and Catalysis,
Faculty of Chemistry, University of Bucharest,
4-12, Blv. Regina Elisabeta, 030018 Bucharest, Romania
e-mail: ioancezar.marcu@g.unibuc.ro;
ioancezar_marcu@yahoo.com
´
E. Mako
´
Institute of Materials Engineering,
Faculty of Engineering, University of Pannonia,
´
10 Egyetem St, 8201 Veszprem, Hungary
In the present study, the esterification of n-butanol with
acetic acid, without adding a catalyst and heterogeneously
catalyzed with alumina and alumina-supported molybde-
num oxides, has been studied. The effects of the reaction
time, the molar ratio between the reactants, the speed of
´
A. Redey
´
Department of Environmental Engineering and Chemical
Technology, Faculty of Engineering, University of Pannonia,
´
10 Egyetem St, 8201 Veszprem, Hungary
123

Esterification of Acetic Acid with n-Butanol
33
agitation and the catalyst loading on the reaction were
investigated.
hotplate. The reaction was carried out at 100 °C with a
molar quantity of acetic acid of 0.09 and an n-butanol-to-
acetic acid molar ratio varied from 1 to 3. Cyclohexane
¨
(Riedel-de Haen, 99.5%) was always added to the reaction
2 Experimental
mixture for water removal, the cyclohexane-to-acetic acid
molar ratio being kept at 1. The amount of catalyst was
varied between 0.5 and 1.0% of the mass of mixture charge
in the reaction and the reaction time varied between 30 and
120 min. Unless specified, all experiments were conducted
at a speed of agitation of 600 rpm. All the catalysts used in
the reaction were in the powder form. Samples from the
organic layer were withdrawn at regular intervals and
analyzed with a Thermo Finnigan chromatograph using a
DB-5 column and a flame ionization detector. Under the
employed conditions of reaction, both in the presence and
in the absence of a catalyst, butyl acetate was the only
product detected. The mass balances, calculated after a
reaction time of 120 min, were always higher than 94%.
2.1 Catalyst Preparation and Characterization
Al2O3 support was prepared from Al(NO3)3Á9H2O (Fluka
Analytical) by precipitation with ammonium carbonate
(Lachema) at controlled pH of 6.5. MoO3 was introduced
at two concentrations, 5% and 10% by weight, via incipient
wetness impregnation of the alumina support with aqueous
(NH4)6Mo7O24Á4H2O (Fluka Analytical) solutions contain-
ing appropriate amounts of molybdenum. After impreg-
nation, the samples were dried in air at 100 °C and then
calcined at 600 °C for 4 h. The 5 wt% MoO3/Al2O3 and
10 wt% MoO3/Al2O3 samples were labelled 5Mo–Al2O3
and 10Mo–Al2O3, respectively.
The crystalline phases were investigated by the X-ray
diffraction (XRD) method. XRD patterns were obtained
with a Philips PW 3710 type diffractometer equipped with
3 Results and Discussion
˚
a CuKa source (k = 1.54 A), operating at 50 kV and
3.1 Catalysts Characterization
40 mA. They were recorded over the 10–70° angular range
with 0.02° (2h) steps and an acquisition time of 1 s per
point. Data collection and evaluation were performed with
PC-APD 3.6 and PC-Identify 1.0 software.
Figure 1 shows the XRD patterns of the supported catalysts
and the alumina support. Only broad lines corresponding to
c-alumina (PDF 10-425) were observed in all cases sug-
gesting that MoOx species are well dispersed on the alu-
mina support.
Surface areas of the catalysts were measured from the
adsorption isotherms of nitrogen at 77 K using the BET
method with a Micromeritics ASAP 2020 sorptometer.
The acidity of the catalysts was estimated by tempera-
ture-programmed desorption of ammonia (NH3-TPD).
About 0.1 g of the catalyst sample was dehydrated at
500 °C in dry air for 1 h and purged with N2 for 0.5 h. The
sample was then cooled down to 100 °C under the flow of
N2, and NH3 was supplied to the sample until its saturation.
For the desorption of the physisorbed ammonia, a nitrogen
stream was passed over the sample, at the same tempera-
ture, until no more NH3 was observed in the exit flow.
Finally, the chemisorbed NH3 was desorbed in a N2 flow by
increasing the temperature up to 500 °C with a heating rate
of 10 K/min. The ammonia desorbed was bubbled through
a solution of sulfuric acid. The acid in excess was titrated
with a solution of NaOH, the amount of ammonia desorbed
being then calculated.
The surface areas and the pore volumes of the catalysts
are given in Table 1. The surface area and the total pore
volume of the samples decreased by dispersing MoO3 on
alumina and by increasing its content. The reduction in
surface area and pore volume has already been observed
for alumina-supported molybdenum oxide [10] and may be
due to the blockage of pores by MoOx species. The
supported samples possess high surface areas due to the
dispersion effect of porous carrier.
A key point to understand the catalytic behavior in an
acid-catalyzed reaction deals with the identification of the
acid function of the materials. This was achieved using the
NH3-TPD technique. The total acidities of the catalysts,
expressed as the total number of acid sites per gram of
catalyst, determined by NH3-TPD, are presented in
Table 1. It can be observed that the total acidity is higher
for the supported samples than for the alumina support and
increases with increasing the MoO3 loading. Nevertheless,
we note that the desorption of all the adsorbed ammonia
took place at temperatures lower than 350 °C in the case of
both supported molybdenum oxide samples while in the
case of alumina support the temperature has been raised up
to 500 °C for the desorption of all the adsorbed ammonia.
This suggests that the supported molybdenum oxide
2.2 Esterification Reactions
The esterification reactions of acetic acid (Chimactiv,
¨
99.5%) with n-butanol (Riedel-de Haen, 99.5%) were
performed in a 150 mL two-neck flask equipped with a
condenser and an additional port for sample withdrawal.
The above assembly was heated using a thermostated
123

34
G. Mitran et al.
70
60
50
40
30
20
10
0
10Mo-Al
2O3
5Mo-Al2O3
0
30
60
90
120
150
Reaction time (min.)
Al2O3
Fig. 2 Conversion of acetic acid as a function of the reaction time in
the catalytic and non-catalytic reaction. Reaction conditions: molar
ratio acetic acid:n-butanol = 1:1, reaction temperature 100 °C,
0.7 wt% catalyst. (circle non-catalytic reaction, diamond Al2O3,
triangle 5Mo–Al2O3, square 10Mo–Al2O3)
10
20
30
40
50
60
70
2-theta (degrees)
in the absence of a catalyst is shown in Fig. 2. As expected,
the conversion of acetic acid was higher in the presence of
a catalyst than without a catalyst and increased remarkably
with the prolonging of the reaction time. The activities at
120 min of reaction time are compared in the following. As
known, esterification is a typical acid catalyzed reaction,
and the acidity of the catalyst would exert a great influence
on the catalytic activity. The highest conversion (62.6%) of
acetic acid over 10Mo–Al2O3 catalyst may thus be due to
its high acid site density (Table 1) comparatively with
5Mo–Al2O3 (conversion 41.4%) and Al2O3 (conversion
32.2%) catalysts that have lower acid site densities. Nev-
ertheless, when the site-time yields (STY, mol/mol H?/s),
are compared, the alumina support shows an activity higher
(STY = 0.246 mol/mol H?/s) than 5Mo–Al2O3 and
10Mo–Al2O3 samples for which STY are 0.105 and
0.110 mol/mol H?/s, respectively. This is in line with the
fact that c-alumina possesses stronger acid sites than
the supported molybdenum oxide samples, as shown from
the NH3-TPD experiments.
Fig. 1 X-ray diffraction patterns of the alumina support and of the
alumina-supported MoO3 catalysts
Table 1 Physico-chemical properties of the catalysts
Catalyst
BET surface
area (m2/g)
Total pore
volume (cc/g)
Total acidity
(mmol/g)
Al2O3
227
196
178
0.38
0.35
0.30
0.12
0.36
0.52
5Mo–Al2O3
10Mo–Al2O3
samples exhibit only weak and medium strength acid sites
while alumina support also exhibits high strength acid sites.
3.2 Catalytic Activity
3.2.1 Influence of the Reaction Time
The influence of the reaction time was studied in the fol-
lowing reaction conditions: 8 mL of n-butanol, 5 mL of
acetic acid and 10 mL of cyclohexane; the catalyst repre-
sents 0.7% of the mass of mixture charge in the reaction;
the reaction temperature was kept at 100 °C. In these
conditions, the selectivity of n-butyl acetate was, in all
cases, 100%. This means that the conversion of acetic acid
can represent the yield of n-butyl acetate.
3.2.2 Influence of the Molar Ratio of Reactants
The effect of mole ratio of reactants on the esterification of
acetic acid with n-butanol for catalytic and non-catalytic
reactions is presented in Fig. 3. For all acetic acid to
n-butanol mole ratios, the only product observed was
n-butyl acetate. With the increase in n-butanol to acetic
acid mole ratio from 1:1 to 2:1 and to 3:1, the conversion
increased in all cases because, as expected based on the Le
The conversion of acetic acid measured in the above
reaction conditions as a function of the reaction time over
Al2O3, 5Mo–Al2O3 and 10Mo–Al2O3 catalysts as well as
123

Esterification of Acetic Acid with n-Butanol
35
(b)
(a)
50
50
40
30
20
10
0
30 min
30 min
60 min
90 min
120 min
60 min
40
30
20
10
0
90 min
120 min
1:1
1:2
1:3
1:1
1:2
1:3
Acid to alcohol mole ratio
Acid to alcohol mole ratio
(c)
(d)
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
30 min
30 min
60 min
90 min
120 min
60 min
90 min
120 min
1:1
1:2
1:3
1:1
1:2
1:3
Acid to alcohol mole ratio
Acid to alcohol mole ratio
Fig. 3 Effect of mole ratio on the esterification of acetic acid with n-butanol in the non-catalytic reaction (a) and using Al2O3 (b), 5Mo–Al2O3
(c) and 10Mo–Al2O3 (d) as catalysts. Reaction conditions: temperature 100 °C, 0.7 wt% catalyst
20
ˆ
Chatelier principle, the increase of the alcohol amount
enhances the conversion of the acid. The conversion of
acetic acid reached 81% in the presence of 10Mo–Al2O3
catalyst for an n-butanol to acetic acid mole ratio equal to
3:1 and a reaction time of 120 min.
15
10
5
3.2.3 Influence of the Speed of Agitation
The influence of the speed of agitation on the conversion of
acetic acid was carried out for Al2O3 and 10Mo–Al2O3 cat-
alysts samples at three different speeds 450, 600 and 750 rpm
to examine the influence of an external mass transfer effect
under the following reaction conditions: n-butanol to acetic
acid mole ratio was 1:1, 0.7 wt% catalyst, reaction tempera-
ture 100 °C and reaction time 60 min. The results obtained
are shown in the Fig. 4. It is clear from the figure that for both
catalysts there was no effect of the speed of agitation in the
range 450–750 rpm on the conversion of acetic acid. This
implies that there was no resistance to mass transfer of acid to
the external surface of the catalyst.
0
400
500
600
700
800
Speed of agitation (rpm)
Fig. 4 Effect of the speed of agitation on the conversion of acetic
acid for Al2O3 (diamond) and 10Mo–Al2O3 (square) catalysts.
Reaction conditions: molar ratio acetic acid:n-butanol = 1:1, reaction
temperature 100 °C, 0.7 wt% catalyst, reaction time 60 min
123

36
G. Mitran et al.
3.2.4 Influence of the Mass Fraction of the Catalyst
Table 2 Reusability of the 10Mo–Al2O3 catalyst in the esterification
of acetic acid with n-butanol
The effect of the catalyst concentration on the esterifica-
tion of acetic acid with n-butanol for Al2O3 and 10Mo–
Al2O3 samples at 60 and 120 min is shown in Fig. 5. The
mass fraction of the catalyst was 0.5, 0.7 and 1.0% of the
total reaction mixture. It can be observed, in all the cases
studied, an increase of the conversion of acetic acid with
the mass fraction of the catalyst, more attenuated when
the mass fraction of the catalyst was increased from 0.7 to
1%. The conversion tends to a plateau for mass fractions
of the catalyst around 1%. This means that the optimal
mass fraction of the catalyst in the reaction medium is
around 1%.
Cycle
First
Reaction time (min) Conversion (%) Selectivity (%)
30
60
3.9
17.7
43.4
62.6
3.8
100
100
100
100
100
100
100
100
100
100
100
100
90
120
30
Second
Third
60
18.0
44.5
61.6
4.1
90
120
30
60
17.6
43.8
62.3
90
120
Reaction conditions: n-butanol to acetic acid mole ratio = 1:1,
0.7 wt% catalyst and reaction temperature 100 °C
(a)
60
3.2.5 Reusability of the Catalyst
50
40
30
20
10
0
The used 10Mo–Al2O3 catalyst in the first cycle of the
reaction was separated by filtration and dried in an oven at
120 °C overnight. It was then used for the esterification of
acetic acid with n-butanol with a fresh reaction mixture
under the following reaction conditions: n-butanol to acetic
acid mole ratio equal to 1:1, 0.7 wt% catalyst and reaction
temperature 100 °C. The procedure was repeated after the
second cycle. The results obtained are presented in
Table 2. From these results, it can be concluded that the
catalyst can be reused as there is no loss in catalytic
activity and product selectivity during the three cycles.
0.4
0.6
0.8
1
Mass fraction of the catalyst (%)
4 Conclusion
(b)
90
80
70
60
50
40
30
20
10
0
Molybdenum oxide supported on c-alumina acts as an effi-
cient stable solid acid catalyst for the esterification of acetic
acid with n-butanol. In all the esterification reactions the
selectivity was 100% while the conversion reached about
81% in the presence of 10Mo–Al2O3 sample within 120 min
of reaction time for an n-butanol to acetic acid mole ratio
equal to 3 at 100 °C. The conversion increased in the fol-
lowing order: Al2O3 \ 5Mo–Al2O3 \ 10Mo–Al2O3 in line
with the total acid site density measured by NH3-TPD. The
optimal mass fraction of the catalyst in the reaction medium
was around 1%. The catalytic properties of the 10Mo–Al2O3
catalyst are maintained after three successive reactions.
0.4
0.6
0.8
1
Mass fraction of the catalyst (%)
References
Fig. 5 Effect of catalyst concentration on the esterification of acetic
acid with n-butanol using Al2O3 (a) and 10Mo–Al2O3 (b) as catalysts.
Reaction conditions: temperature 100 °C, molar ratio acetic
acid:n-butanol =1:3. (diamond 60 min, square 120 min)
1. Kirk-Othmer (1994) Encyclopedia of chemical technology, 4th
edn., vol 9. Wiley, New York, p 755
123

Esterification of Acetic Acid with n-Butanol
37
2. Sawant DP, Vinu A, Justus J, Srinivasu P, Halligudi SB (2007)
J Mol Catal A Chem 276:150
8. Zhang F, Wang J, Yuan C, Ren X (2006) Sci China Ser B Chem
49:140
3. Liu Y, Lotero E, Goodwin JG Jr (2006) J Catal 242:278
4. Izci A, Bodur F (2007) React Funct Polym 67:1458
5. Corma A, Garcia H, Iborra S, Primo J (1989) J Catal 120:78
6. Zhang HB, Zhang BZ, Li HX (1992) J Nat Gas Chem 49:56
7. Timofeeva MN, Matrosova MM, Il’inich GN, Reshetenko TV,
Avdeeva LB, Kvon RI, Chuvilin AL, Budneva AA, Paukshtis
EA, Likholobov VA (2003) Kinet Catal 44:778
9. Bhorodwaj SK, Pathak MG, Dutta DK (2009) Catal Lett 133:185
10. Chen K, Xie S, Bell AT, Iglesia E (2001) J Catal 198:232
123