Zinc oxide as a solid acid catalyst for esterification reaction

Article abstract of DOI:10.1016/j.catcom.2015.02.009

Zinc oxide and its composites with Hβ zeolite were produced under different synthesis conditions. The synthesized catalysts were evaluated for performance in esterification of n-butanol with acetic acid. BET surface area, XRD, and acidity (using NH₃ and pyridine as probe molecules) were measured to correlate the activity with the structural features of the catalysts.

Full text of DOI:10.1016/j.catcom.2015.02.009

Zinc oxide as a solid acid catalyst for esterification reaction
Anjani Nagvenkar, Sajo Naik, Julio Fernandes
PII:
DOI:
Reference:
S1566-7367(15)00068-0
doi: 10.1016/j.catcom.2015.02.009
CATCOM 4226
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
12 January 2015
4 February 2015
5 February 2015
Please cite this article as: Anjani Nagvenkar, Sajo Naik, Julio Fernandes, Zinc oxide
as a solid acid catalyst for esterification reaction, Catalysis Communications (2015), doi:
1
0.1016/j.catcom.2015.02.009
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ACCEPTED MANUSCRIPT
Zinc Oxide as Solid Acid Catalyst for Esterification Reaction
Anjani Nagvenkar, Sajo Naik, and Julio Fernandes*
Department of Chemistry, Goa University, Taleigao Plateau, Goa, India, 403206
*
e – mail: julio@unigoa.ac.in
Key words: zinc oxide; solid acid; heterogeneous catalysis; kinetics; esterification.
1
. Abstract
Zinc oxide and its composites with Hβ Zeolite were produced under different synthesis
conditions. The synthesized catalysts were evaluated for performance in esterification
of n-butanol with acetic acid.BET surface area, XRD, and acidity (using NH and
3
pyridine as probe molecules) were measured to correlate the activity with structural
features of the catalysts.
2
. Introduction
Solid acids have a tendency to donate a proton or to accept an electron pair in
heterogeneous acid catalysis reactions such as alkylation, acylation, etc. Esterification
is an important reaction in chemical industry where in an efficient acid catalyst is
required to obtain good yields of ester at higher selectivity [1]. Esterification reaction
can also be performed using homogenous catalysts such as H SO , HF, H PO , but
2
4
3
4
use of such catalysts often causes serious issues in product separation and catalyst
reuse and disposal [2].
The replacement of mineral acids by solid acids (metal oxides, zeolites, clays) has been
a major research area as there is growing demand for environmentally benign
processes using less hazardous chemicals [3,4]. The strong interest in the use of solid
acid catalyst as a replacement to homogenous catalyst is because of their low
corrosiveness, and the ease of regeneration and reuse. ZnO has been shown to have
both acid and base characteristics which vary upon the method of preparation. Already,
pure ZnO and ZnO based catalysts have been used for esterification and trans-
esterification reactions [5-7]. The present investigation is aimed to develop a pure ZnO
or ZnO based solid acid catalyst for direct esterification of carboxylic acids with alcohols
under solvent-free conditions [8].
2
. Experimental
2
.1 Catalyst Synthesis
2
.1.1Synthesis of ZnO and composite ZnO/Hβ zeolite catalysts
1

ACCEPTED MANUSCRIPT
A series of ZnO samples were prepared by using zinc nitrate as a precursor in the
presence of urea and oxalic acid [8]. In general, a known amount of the three reactants
were weighed in a molar ratio of 1: X: Y and designated as Z1XY where 'X' is moles of
urea and 'Y' is moles of oxalic acid. At first, calculated quantities of urea and oxalic acid
were dissolved in a minimum amount of water and this solution was added dropwise to
the zinc nitrate solution under stirring. The obtained precipitate was evaporated to
dryness and then calcined at 450˚C for 4 h in air. Similarly, ZnO/Hβ composite were
prepared by performing precipitation over Hβ zeolites suspended in water.
2
2
.2 Catalyst Characterisation
.2.1 X-ray diffraction(XRD)
XRD patterns were recorded on a Rigaku Ultima IV diffractometer, using Ni filtered Cu
o
Karadiation (=1.5406A ).
2
.2.2 Temperature programmed desorption of ammonia (TPD-NH ) and pyridine
3
adsorption studies
TPD-NH experiments were performed on a Chemisorb 2720, Micromeritics instrument.
3
Typically, in a quartz tube, 25 mg of the sample was used for analysis. The sample was
o
o
activated at 120 C, 12 h. After cooling to 25 C, dry ammonia gas (20 % NH balance
3
He (UHP); 20 mL/min) was passed over the sample for 15 minutes. The physically
o
adsorbed ammonia was flushed out by He carrier gas at 25 C. The sample was heated
o
o
o
from 25 C to 600 C at a heating rate of 5 C/min and the desorbed ammonia was
measured using a thermal conductivity (TC) detector connected to the sample tube in a
loop.The number and strength of acid sites in the catalyst were evaluated based on this
measurement [14].
2
.2.3 Pyridine adsorption studies
The nature of surface acidity of the samples was determined by FTIR study. About 0.1
g of the samples were activated at 120˚C for 1hr prior to the experiment and exposed to
pyridine vapours for 4-5 hrs. The samples were dried at 110˚C, degassed and
FTIRspectra of each were recorded on Shimadzu FTIR instrument.
2
.2.4 BET surface area measurements
The BET surface areas of the samples were measured on a NOVA 2000e,
Quantachrome surface area and porosity instrument using N as a probe molecule at
2
7
7.3 K. The samples were appropriately pre-treated and degassed to clean the
surfaces from moisture and volatiles. The multipoint Brunauer, Emmett and Teller (BET)
method was used to measure total surface area.
2

ACCEPTED MANUSCRIPT
2
.3 Esterification of acetic acid with n-butanol
Typically, 25 ml of glacial acetic acid was taken in a three necked flask and heated to
5˚C. After the desired temperature has been reached, 20 ml of n-butanol preheated
separately was added into the reactor flask. The acetic acid : n-butanol molar ratio was
:1 in all the experiments. An amount of 50 mg of catalyst was used in each case.
9
2
Aliquots of the reaction mixture were analysed titrimetrically at regular time intervals to
estimate the % conversion [9]. % Conversion of acetic acid = [C - C /C] x 100 where C
i
i
t
i
is in initial concentration of acetic acid and C is concentration after time t.
t
3
. Results and Discussion
3
.1. Synthesis and structural properties of catalysts
The XRD patterns (not shown here) confirmed the formation of ZnO from the
observation of (100), (002), (101), (102) and (110) peaks indexed on the basis of
hexagonal wurtzite [JCPDS no. 36-1451]. The acid-base characteristics of a solid acid
catalyst such as ZnO has been established earlier [6,8,10]. ZnO/SiO is recently
2
investigated for esterification of fatty acids, mediated by acid sites [11]. As solid state
properties and catalytic reactivity of ZnO are known to depend upon the method of
preparation [9], in this investigation, ZnO samples were prepared by calcination of zinc
nitrate with varying proportions of urea and oxalic acid.
Table 1 shows the list of prepared samples along with the corresponding surface areas
and the activity in esterification reaction.It can be seen from Table 1 that the surface
2
-1
areas of zinc oxides ranged from 2-30 m g . The average particle sizes of Z-102, Z-
22, Z-124 were calculated by the Scherrer equation was ~34 nm. Z-100 showed
1
significant value of surface area due to large crystallite size. The surface areas
increased for Z-102 due to calcination in presence of oxalic acid. Further the sample Z-
1
22 prepared by using urea in addition to oxalic acid, significantly increased the surface
2
2
area from ~19 m /g to 29 m /g. The higher surface area resulted due to the porogenic
properties of urea and oxalic acid. Presence of urea resulted in ZnO particles being of
lower crystallite size, as well as enhanced porosity and surface area [8]. Z-124 catalyst
was prepared in order to investigate the effect of excess oxalic acid during the
calcination step, particularly because in the preceding sample Z-122, the presence of
oxalic acid had resulted in enhanced surface area. However it was observed that
excess oxalic acid did not cause further enhancement in surface area. On the contrary
2
the surface area was significantly lower and was only about 17 m /g. This effect is
interpreted from the point of view that excess oxalic acid acted as a fuel particularly in
presence of urea and resulted in local rise in temperatures, causing sintering of some
particles with a consequent decrease in surface area.
3

ACCEPTED MANUSCRIPT
Table 1: Percentage conversion of acetic acid during esterification reaction on various
zinc oxide catalysts in relation to their surface area.
Sample
Code
Mole ratios of reagents
%
Conversion
of acetic
acid
BET
Surface
area (m /g)
Zn(NO3)2 Urea Oxalic acid
2
Z-102
Z-122
Z-124
1
1
1
0
2
2
2
2
4
19
29
17
23
42
31
Z-100
1
0
0
3
9
ZHβ-122
ZHβ-124
1
1
2
2
2
4
-
-
37
29
3
.2. Reactivity of pure ZnO catalyst
The esterification activity in terms of % conversion of acetic acid and in relation to the
number of acid sites is presented in Table 1. The reported conversions are equilibrium
conversions. In all the experiments the equilibrium was reached at the end of ~ 2 hours.
The catalyst Z-100 expectedly showed a minimum conversion of acetic acid (< 10 %).
Furthermore, the catalytic activity showed dependence on surface area. Thus, as seen
in Table 1, in comparison to Z-100, Z-102 showed a much higher conversion of 23.%.
The activity of Z-122 was the highest with 42%. This was in agreement with its large
2
surface area of 29 m /g. However, only ~ 31 % conversion was observed in case of Z-
1
24 catalyst.
Although comparing Z-102 and Z-124 we see that the surface area is almost similar
2
(
~18 m /g), their catalytic activity was widely different. Thus surface area was not the
only criterion to determine the catalytic activity. In conclusion the activity of the various
zinc oxide catalysts were in the following order : [Z-122] > [Z-124] >[ Z-102] > [Z-100].
Since surface areas of the catalysts were not solely responsible for the esterification
activity, the cause of enhanced activity of Z-124 as compared to Z-102 was explained
considering the TPD profiles shown in Fig. 1.Z-100 showed a weak TPD profile
indicating near absence of acid sites as compared to the other samples prepared with
use of oxalic acid and/or urea. These later samples showed two distinct peaks in the
temperature range 50 - 400˚C which may be classified as low temperature (150 -
2
00˚C) and medium temperature (200-350˚C) peaks. The magnitude of these peaks
followed order:
Low Temperature (LT)
: [Z-122] > [Z-102] > [Z-124]
Medium Temperature (MT) : [Z-122] > [Z-124] > [Z-102]
4

ACCEPTED MANUSCRIPT
Table 2: Percentage conversion of acetic acid during esterification reaction on various
zinc oxide catalysts in relation to strength of their acid sites.
Sample
Total
acidity
mmol/g)
Strength of acid sites
(mmol/g)
%
conversion
of acetic
acid
(
Weak
RT -
50˚C)
Medium
(150˚C -
300˚C)
Strong
(300˚-
400˚C)
(
1
Z-122
0.952
0.720
0.696
1.146
0.985
0.360
0.232
0.344
0.299
0.220
0.457
0.335
0.288
0.587
0.459
0.135
0.157
0.064
0.258
0.305
42
31
23
37
29
Z-124
Z-102
ZHβ-122
ZHβ-124
Fig 1. TPD Profiles of different ZnO catalysts.
5

ACCEPTED MANUSCRIPT
It is thus clear from Table 2 that the percent conversion of acetic acid can be directly
correlated to the concentration of acid sites of medium strength. Thus Z-122 with
highest concentration of medium strength acid sites showed maximum percent
conversion.
Also Z-124 showed higher conversion than Z-102 because of increased number of
medium strength acid sites and not necessarily the factor of surface area. Therefore it
was inferred that this esterification reaction was essentially a moderate acid site
dependent reaction. Solid zinc oxide prepared in this work could be one of the most
desirable catalyst due to its high selectivity, reusability, activity and less corrosivity [12].
3
.3. Reactivity of ZnO in relation to Hβ-zeolites.
Protonic zeolites are well known solid acid catalysts [13]. To assess the comparative
potential of the synthesized ZnO catalysts, the reaction was also performed under
similar conditions for pure Hβ zeolite and on ~15% ZnO supported on Hβ zeolite. Pure
Hβ zeolite showed an activity of ~47% . This was a little higher than 41% activity of Z-
1
22 that was obtained by a simple one step calcination method. It was therefore felt the
ZnO-Hβ composite catalyst could possibly produce a synergistic effect to show higher
activity.Hence the two additional catalysts Z-122(Hβ) and Z-124 (Hβ) with 15 %
component of ZnO were prepared. These catalysts were designated as ZHβ-122 and
ZHβ-124. When these two composite catalysts were used for esterification, contrary to
expectation they showed lower activity of 37 % for ZHβ-122 and 29 % for ZHβ-124 than
either of the pure catalyst ZnO or Hβ-zeolite. Hence it was concluded that the
mechanisms of esterification could be different on Hβ zeolite and ZnO catalysts. Further
investigation was carried out to elucidate the nature of acid sites by pyridine adsorption
studies using FTIR as shown in Fig. 2.
It is well known that adsorbed pyridine shows characteristic IR absorptions at
-1
-1
~
1450 cm and 1510 cm due to presence of Lewis and Bronsted acid sites
-1
respectively. The peak at 1490 cm is a combination band due to Lewis and Bronsted
acid sites. It can be seen from the figure that the ZnO samples (Z-102, Z-122, Z-124)
showed the above absorptions due to the presence of both Lewis and Bronsted sites.
Closer examination of these spectra suggests that the active sample Z-122 is rich in
Lewis acid sites as compared to the other catalysts. Thus it is proposed that Lewis sites
are preferable active centres on ZnO in esterification reactions [11,14]. The Hβ zeolite
is predominantly Bronsted acid catalyst and esterification with these catalysts was
primarily mediated through protons as active centres by protonation of the substrate
leading to formation of ester. The lower activity of ZnO(Hβ) samples could be due to
decrease in availability of protons as active centres [15].
Our reaction kinetics study indicated operation of Eley-Rideal mechanism (Fig. 3) for
2+
esterification on ZnO considering pre-adsorption of the carboxylic acid with Zn acting
as Lewis acid centres. All the esterification reactions in the present work were carried
out with an acetic acid to n-butanol molar ratio of 2:1. Graphs were plotted of ln C v/s t
t
6

ACCEPTED MANUSCRIPT
and 1/C v/s t where C is equivalent concentration of acetic acid after time t. From
t
t
2
the resulting kinetic plots, it was observed that the correlation coefficient R term for the
first order reaction gave a better fit than the correlation coefficient for second order
reaction. Thus the average R values using the ZnO catalysts for esterification were
2
0
.95 for first order and 0.84 if we considered second order kinetics. It was therefore
concluded that the esterification of acetic acid and n-butanol followed pseudo first order
reaction kinetics. Hence Eley-Rideal mechanism was proposed. Eley – Rideal
mechanism was also recently proposed on ZrO catalyst [16].
2
Fig. 2 : FT-IR spectra of pyridine adsorbed zinc oxide catalysts.
7

ACCEPTED MANUSCRIPT
Fig. 3 :Proposed Eley-Rideal mechanism for esterification of acetic acid with n-butanol
over zinc oxide catalyst.
4
. Conclusions
Pure ZnO and ZnO/Hβ zeolites were synthesized and utilized as solid acid catalysts for
esterification of acetic acid with n-butanol. ZnO synthesized by calcination of zinc nitrate
in presence of urea and oxalic acid in molar ratio of 1:2:2 showed a high rate of
esterification (% conversion of ~42 %) comparable to Hβ zeolite ~46%.
5
. Acknowledgements
The authors are grateful to the Department of Science & Technology, New Delhi (India),
for financial support under Nanomission project (SR/NM/NS-86/2009).
6
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[
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Highlights



Synthesized ZnO is active in n-butanol esterification reaction
The activity of ZnO is comparable to that of Hβ Zeolite
A reaction mechanism is provided from kinetics data
1
0