Transesterification of diethyl malonate with benzyl alcohol catalyzed by modified zirconia: Kinetic study

Article abstract of DOI:10.1016/j.molcata.2014.03.025

Zirconia and its modified forms such as 10%Mo(VI)/ZrO₂, 10%V(V)/ZrO₂, 10%W(VI)/ZrO₂ and SO₄ ^2-/ZrO₂ were prepared and characterized for their physico-chemical properties such as BET for surface area, NH₃-TPD and n-butylamine back titration method for total surface acidity, PXRD technique for crystallinity and ICP-OES technique for elemental analysis. These materials were used as catalysts in liquid phase transesterification reaction of diethyl malonate (DEM) with benzyl alcohol (BA). Optimization of reaction conditions such as reaction time, reaction temperature, weight of the catalyst and molar ratio of reactants were carried out to obtain highest possible transester yield. Dibenzyl malonate (DBM) and benzyl ethyl malonate (BEM) were obtained as major products. Highest total transester yield of (88%) was obtained in the presence of 0.75 g of SZ catalyst at a molar ratio of DEM: BA = 1:3, reaction temperature of 393 K and reaction time 5 h. Kinetic studies were carried out to find out the rate of the reaction and energy of activation values for zirconia catalysts, in order to identify a facile catalyst system for this reaction. A possible reaction mechanism was proposed based on the kinetic data and it was observed that Eley-Rideal mechanism fits well for this reaction. Reactivation and reusability studies of the catalysts were also taken up.

Full text of DOI:10.1016/j.molcata.2014.03.025

Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Transesterification of diethyl malonate with benzyl alcohol catalyzed
by modified zirconia: Kinetic study
N. Thimmaraju, S.Z. Mohamed Shamshuddin∗, S.R. Pratap, Venkatesh
Chemistry Research Laboratory, HMS Institute of Technology, Tumkur 572104, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Zirconia and its modified forms such as 10%Mo(VI)/ZrO2, 10%V(V)/ZrO2, 10%W(VI)/ZrO2 and SO42−/ZrO2
were prepared and characterized for their physico-chemical properties such as BET for surface area, NH3-
TPD and n-butylamine back titration method for total surface acidity, PXRD technique for crystallinity
and ICP-OES technique for elemental analysis. These materials were used as catalysts in liquid phase
transesterification reaction of diethyl malonate (DEM) with benzyl alcohol (BA). Optimization of reaction
conditions such as reaction time, reaction temperature, weight of the catalyst and molar ratio of reactants
were carried out to obtain highest possible transester yield. Dibenzyl malonate (DBM) and benzyl ethyl
malonate (BEM) were obtained as major products. Highest total transester yield of (88%) was obtained
in the presence of 0.75 g of SZ catalyst at a molar ratio of DEM: BA = 1:3, reaction temperature of 393 K
and reaction time 5 h. Kinetic studies were carried out to find out the rate of the reaction and energy
of activation values for zirconia catalysts, in order to identify a facile catalyst system for this reaction.
A possible reaction mechanism was proposed based on the kinetic data and it was observed that Eley-
Rideal mechanism fits well for this reaction. Reactivation and reusability studies of the catalysts were
also taken up.
Article history:
Received 24 August 2013
Received in revised form 24 March 2014
Accepted 25 March 2014
Available online 18 April 2014
Keywords:
Modified zirconia
Transesterification
Dibenzyl malonate
Benzyl ethyl malonate
Kinetics and mechanism
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Some modified oxides exhibit strong surface acidity due to gen-
eration of large number of positive or negative charge in the binary
Solid acids are safe and eco-friendly than liquid acids such as,
H2SO4, HCl, HF, AlCl3, BF3, ZnCl2 and SbF5 due to their specific
features such as non-toxic, non-corrosive, easy to handle, easily
recoverable, reusable and less expensive. Various solid acids such
as zeolites, cation exchange resins, metal oxides and their modi-
fied forms, doped metal oxides, etc., have been successfully used
as catalysts for many acid catalyzed organic transformations such
as isomerization [1], dehydration [2], acylation [3], oxidation [4],
transesterification [5,6], esterification reaction [7], etc.
Now-a-days metal oxide-based catalysts are active over a wide
range of temperatures and having high resistance to thermal excur-
sions. Over the past few years, the preparation, characterization and
advantages of zirconia based solid acids have gained much atten-
tion in various organic transformations due to their non-toxicity,
super acidity, simplified product isolation, high selectivity, reduc-
tion of wasteful byproducts and high activity at low temperatures
[8–10].
oxide model structure. Modified forms such as SO42−, Mo(VI), V(V)
and W(VI) supported on ZrO2 exhibit strong acidity and excellent
catalytic activity towards various organic transformations in the
liquid phase [5,6,11–17].
Esters, which are having wide range of category in organic
compounds ranging from aliphatic to aromatic, are important inter-
mediates in the synthesis of drugs, food preservatives, plasticizers,
fine chemicals, cosmetics, perfumery, solvents, chiral-auxiliaries,
agro chemicals and also as precursors for a number of pharma-
ceutical products. Especially, malonic esters are known for their
importance either as starting materials or intermediates in organic
fine chemical synthesis [18,19].
Keeping in view the importance of zirconia based solid acids and
benzyl malonate esters, the goal of the present work has been set
to explore the use of solid acids such as zirconia and its modified
forms like Mo(VI)/ZrO2 (MZ), V(V)/ZrO2 (VZ), W(VI)/ZrO2 (WZ) and
SO42−/ZrO2 (SZ) in acid catalyzed transesterification of DEM with
BA. The total transester yield and percentage selectivity towards the
desired product was optimized by varying reaction time, reaction
temperature, weight of the catalyst and molar ratio of the reactants.
In order to find out the best catalyst for transesterifation of DEM
with BA among those used in the present study, kinetic studies
∗
Corresponding author. Tel.: +91 9844718742.
E-mail address: mohamed.shamshuddin@gmail.com
(S.Z. Mohamed Shamshuddin).
http://dx.doi.org/10.1016/j.molcata.2014.03.025
1381-1169/© 2014 Elsevier B.V. All rights reserved.

56
N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
were carried out to find out the data such as rate of the reaction
and energy of activation.
magnetic stirrer. The total volume of the reaction mixture was
kept constant at 10 ml. After a definite period of time the reac-
tion mixture was cooled, filtered and the filtrate was analyzed by
gas chromatograph (GC) fitted with 10% carbowax column coupled
with flame ionization detector and qualitatively by LCMS (Agilent).
To optimize the reaction conditions, parameters such as reaction
time, reaction temperature, weight of the catalyst and molar ratio
of the reactants were varied. Kinetic studies were conducted in the
reaction time from 1 to 9 h, temperature ranging from 353 to 453 K,
by varying catalyst weight in the range of 0.25–1.00 g and molar
ratio of DEM: BA = 1:2–1:5 (with excess of BA) and with molar ratio
of BA: DEM = 1:0.33–1:2 (with excess of DEM). The effect of nature
of catalyst was also studied to find a facile catalyst desirable for
transesterification of DEM with BA. Kinetic studies were carried
out in presence of these short listed catalysts.
The Langmuir-Hinshelwood (LH) model or an Eley-Rideal (ER)
model is commonly used to correlate the kinetic data for the solid
acid catalyzed reactions [20–22]. These two models are derived
based on the assumption that the rate limiting step is the sur-
face reaction between two adsorbed molecules (LH) or between
an adsorbed molecule and a molecule in the bulk (ER). Attempt has
been made to correlate the kinetic data into LH and ER models and
describe the reaction mechanism for the transesterification of DEM
and BA based on the best fit.
2. Experimental
2.1. Preparation of catalytic materials
2.1.1. Preparation of hydrated ZrO2
2.4. Reusability of catalytic materials
Hydrated zirconia (Zr(OH)4) was prepared by precipitation
method. 100 g of zirconium oxychloride (ZrOCl2·8H2O) was dis-
solved in 250 ml of deionized water and the solution was heated
to 353 K for about 10 min. To this 1:1 aqueous ammonia solution
was added dropwise with constant stirring. Thus obtained precip-
itate of Zr(OH)4 was filtered, washed with deionized water until it
becomes chloride free. Thus obtained precipitate was dried in hot
air oven at 393 K for 12 h.
To study the reusability of the catalytic materials, used catalytic
material was filtered from the reaction mixture, washed with ace-
tone, dried at 393 K for 2 h and calcined at 832 K for 2 h. Thus
obtained reactivated catalyst was subjected to transesterification
of DEM with BA under similar reaction conditions. The re-usability
of re-activated catalysts were carried out for 5 consecutive reaction
cycles.
2.1.2. Preparation of MZ, VZ and WZ
3. Results and discussion
MZ, VZ and WZ were prepared by impregnation method.
Typically, MZ was prepared by making
a paste of 9.1 g of
3.1. Characterization of catalytic materials
zirconyl nitrate (ZrO(NO3)2·H2O) and 0.7 g of ammonium molyb-
date ((NH4)6Mo7O24·H2O) with approximately 10 ml of deionized
water. The paste was dried in hot air oven at 393 K for 12 h and made
a fine powder. Similarly, VZ and WZ were prepared by using zir-
conyl nitrate, ammonium metavanadate and ammonium tungstate.
Zirconia and its modified forms were characterized for their
physico-chemical properties such as surface area, total surface
acidity (along with acid site distribution) and % of metal ions and
the data are given in Table 1.
The results obtained from ICP-OES indicated the modified
forms of zirconia i.e., WZ, VZ and MZ consisted of 10.32%W(VI),
10.59%V(V) and 9.87%Mo(VI) respectively. The BET surface area of
zirconia and its modified forms are presented in Table 1. The surface
area is in the increasing order of ZrO2 < WZ < VZ < MZ < SZ. The sur-
face area of zirconia increases when impregnated with WOx, VOx,
2.1.3. Preparation of sulfated zirconia (SZ)
SZ was prepared by making a fine paste of 10 g of previously
prepared Zr(OH)4 with 6 ml of 3 M H2SO4. The resulting paste was
dried in a hot air oven at 393 K for 12 h and finely powdered.
Thus prepared samples were calcined in a muffle furnace at
832 K for 5 h before their use as catalytic material.
2−
MoOx or SO4 ions. Among these, SZ is found to have higher sur-
2.2. Characterization of catalytic materials
face area which is attributed to the cracking of ZrO2 crystallinity
into fine particles when treatment with sulfate ions [23]. In case
of other modified forms, increase of the surface area is related to
The specific surface area of all the prepared catalysts were
measured by NOVA 1000 Quanta chrome high-speed gas sorption
analyzer instrument. In this analysis, the catalytic material was
degassed at 523 K for 5 h before measurements. Specific surface
area of catalyst was determined by BET conventional method. The
total surface acidity of the catalytic material was measured by NH3-
TPD method by using Plus Chemisorb 2705 from Micromeritics and
also by n-butyl amine back titration method using dry benzene as
a solvent. The zirconia catalysts were characterized for their pow-
der XRD using Xpert Pro Philips diffractometer equipped with a
the formation of M
O Zr (M = W or V or Mo) linkages resulting in
a porous material [24]. The interaction of metal ions with zirconia
also protects from sintering and hence bear the highest surface area.
This phenomenon can again be directly correlated with tetragonal
crystalline phase because tetragonal crystalline size is relatively
smaller than monoclinic phases [25].
The total surface acidity (TSA) as well as acid site distribu-
tion of zirconia and its modified forms measured by NH3-TPD
method are given in Table 1. The TSA of the catalysts are in the
order: ZrO2 < WZ < VZ < MZ < SZ. It also indicates that the impreg-
˚
Ni filtered Cu-K␣ radiation with ꢀ = 1.5418 A using a graphite crys-
2−
tal monochromator in the range of scanning 25–60◦. The amount
of tungsten, vanadium and molybdenum present in WZ, VZ and
MZ catalyst samples was estimated by Inductively Coupled Plasma-
Optical Emission Spectrometer (ICP-OES) analysis technique using
Thermo-iCAP 6000 Series instrument.
nated WOx, VOx, MoOx and SO4 ions have a strong influence on
the acidic properties of zirconia. The TSA values of the catalysts
were also determined by n-butylamine back titration method and
their values are in good agreement with the TSA values obtained
by NH3-TPD method. When the values of acid site distribution of
different zirconia catalysts were compared, pure zirconia consisted
of only ‘weak and medium’ acid sites. Whereas, modified forms of
zirconia i.e., WZ, VZ, and MZ consisted of ‘medium and strong’ acid
sites. However, SZ was found to have ‘strong and very strong’ acid
sites (super acid sites). Further, the acid site distribution values are
in good agreement with the values provided in the literature [26].
2.3. Catalytic activity measurement
The liquid phase transesterification of diethyl malonate (DEM)
with benzyl alcohol (BA) in presence of catalytic material was
carried out in a 25 ml round bottomed flask on a hot plate cum

N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
57
Table 1
Physico-chemical properties of solid acid catalytic materials used in the present study.
Catalyst
BET surface area (m2/g)
Acid site distribution (mmol/g)
Metal (%)
–
10.32 (W)
10.59 (V)
9.87 (Mo)
–
Weak
Medium
Strong
Very strong
TSA
ZrO2
WZ
VZ
MZ
SZ
42.9
44.3
65.6
88.1
161.8
0.03
–
–
–
–
0.40
0.11
0.12
0.16
–
–
–
–
–
–
0.43 (0.45)
0.89 (0.88)
0.95 (0.90)
1.02 (1.03)
1.31 (1.35)
0.70
0.83
0.86
0.87
0.44
Note: Numbers in the parenthesis correspond to the total surface acidity (TSA) values obtained by n-butyl amine back titration method.
The PXRD patterns of zirconia and its modified forms are shown
in Fig. 1. As seen from the figure, pure zirconia has a mixture of
monoclinic and tetragonal phases. However, upon modification
with W(VI) or V(V) or Mo(VI) or SO42− ions, zirconia samples exhib-
ited prominent lines due to tetragonal phase indicating that the
impregnated ions show strong influence on the phase modifica-
tion of zirconia from monoclinic phase to the metastable tetragonal
phase. It is reported that the tetragonal phase of zirconia is more
catalytically active [26].
In the modified form of zirconia increase in the concentration
of tetragonal phase can be attributed to the strong interaction of
W(VI) or V(V) or Mo(VI) or SO42− ions with zirconia which inhibits
the monoclinic phase of zirconia. In case of SZ, the incorporated
sulfate ions may also retard the formation of larger crystallinities
of zirconia and stabilize into metastable tetragonal phase [26–28].
indicates that transesterification of DEM with BA is a catalyzed
reaction.
3.2.1. Effect of nature of zirconia catalyst on the
transesterification reaction
To obtain better yield of transester zirconia was modified with
W(VI) or V(V) or Mo(VI) or SO42− ions and to find a suitable catalyst
among ZrO2, WZ, VZ, MZ and SZ, transesterification of DEM with
BA was carried out under a typical set of reaction conditions. The
obtained results are presented in Table 2. According to the GC–MS
analysis conducted as a part of qualitative analysis, only two prod-
ucts i.e., benzyl ethyl malonate (BEM) and dibenzyl malonate (DEM)
were formed as major products during the reaction.
It was observed that the catalytic activity increased in the
order of ZrO2 < WZ < VZ < MZ < SZ. The catalytic activity of pure ZrO2
exhibited lowest activity for transesterification whereas SZ exhib-
ited excellent activity towards total transester product (% yield of
BEM + % yield of DBM) as well as in the formation of a diester. This
might be due to its higher surface acidity and also possessing the
highest concentration of catalytically active tetragonal phase. It is
also observed that the total transester yield is directly proportional
to the surface acidity of the catalytic material.
Further, the selectivity towards the reaction products (either
BEM or DBM) was found to be related to acid strength of the cat-
alytic material. It is reported that pure zirconia posseses ‘weak and
medium’ acid sites; MZ, WZ and VZ consists of both ‘medium and
strong’ acid sites but SZ posseses ‘strong and very strong’ acid sites
[26,28,29]. When the acid strength of the catalytic material is corre-
lated with the selectivity of BEM or DBM, it can be inferred that the
formation of diester i.e. DBM requires ‘strong and very strong’ acid
sites which are available in SZ catalyst. And the selectivity towards
BEM is more in case of other catalytic material which indicates that
the formation of monoester (BEM) required ‘weak or medium’ acid
sites.
No diffraction peaks corresponding to WOx, VOx, MoOx and
2−
SO4
are observed in the PXRD patterns of WZ or VZ or MZ or
SZ samples. This indicates that in modified form of zirconia WOx,
VOx, MoOx and SO42− ions are satisfactorily incorporated with ZrO2
ions.
Further, it is interesting to note that triangular correlationship
exists between TSA, catalytic activity and crystallinity of the cata-
lysts, i.e., the catalytic material which consisted of more tetragonal
phase was found to be more acidic and further such materials are
found to be more catalytically active. This observation indicates
that the tetragonal phase of zirconia is responsible for higher sur-
face acidity of zirconia catalysts [6,26].
3.2. Catalytic activity studies
Liquid phase transesterification of DEM with BA was carried
out in presence of zirconia and its modified forms as catalysts
(Scheme 1). All the catalysts used in our present work were active
in this reaction.
When the reaction was carried out in the absence of any cat-
alytic material, there was a negligible formation of products. This
3.2.2. Effect of reaction time
The total yield of transester product was examined against the
reaction time by varying reaction time from 1 to 9 h over zirconia,
MZ and SZ catalyst. The obtained results are presented in Fig. 2.
The total yield and selectivity towards DBM gradually increases
with increasing the reaction time and rapidly increases at 3–4 h and
reaches maximum at 6 h over all the catalysts. After 6 h of reaction
T
(e)
T
(d)
Table 2
(c)
T
Activity of different catalysts in the transesterification of DEM with BA. [Reac-
tion conditions: reaction temperature = 433 K, weight of catalyst = 0.5 g, reaction
time = 4 h, DEM = 3.3 ml: BA = 6.7 ml (1:3).].
M
(b)
M
M
Catalyst
Total transester
yield (%)
Selectivity of
BEM (%)
Selectivity of
DBM (%)
T
(a)
ZrO2
WZ
VZ
MZ
SZ
23.5
35.6
38.9
42.0
62.2
69.7
67.1
65.0
62.9
30.4
30.3
32.9
35.0
37.1
69.6
30
40
50
60
2θ (degree)
Fig. 1. PXRD patterns (a) ZrO2, (b) WZ, (c) VZ, (d) MZ and (e) SZ. [M, monoclinic;
and T, tetragonal phase.]

58
N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
O
O
O
O
Acid catalyst
OH
Scheme 1. Transesterification of DEM with BA in presence of an acid catalyst.
O
O
O
O
O
O
+
+
OH
+
O
O
period the total transester yield decreases to an extent of ∼3% and
the selectivity towards DBM was found to increase gradually. Over
all the catalysts the selectivity towards BEM was found to decrease
with reaction time with an increase in the selectivity towards DBM.
The formation of DBM for longer reaction periods can be attributed
to the higher contact time between the reactants and the active
sites of the catalytic material which facilitates further reaction of
the reaction intermediates to produce more DBM. Therefore, 6 h
reaction time was found to be suitable for further optimization of
other reaction conditions. Further, a decrease in the yield of total
transester beyond 6 h of reaction time could be attributed to the
formation of by-products other than BEM and DBM, when the con-
tact time between the catalyst and the reactants/products/reaction
intermediate was increased. Further, a slight change in the colour
of the reaction mixture was also observed after 6 h of the reaction
time.
However, the yield of BEM was found to be increased with a con-
comitant decrease in the yield of DBM.
Since, transesterification is an equilibrium reaction, increase in
the concentration of one of the reactants may facilitate the forward
reaction resulting in the formation of more amount of the prod-
uct. In the present case an increase in the concentration of BA was
found to be desirable to obtain more amount of the desired product.
Therefore, the molar ratio of 1:3 (with excess of BA) was considered
as an optimized molar ratio and used for further studies.
3.3. Kinetic studies
Kinetic studies were carried out over ZrO2, MZ and SZ cata-
lysts. Prior to the kinetic studies, experiments were conducted to
establish the effect of heat and mass transfer limitations during
the transesterification reaction. The heat transfer limitations can be
determined by carrying out experiments at different stirring rates
of the reaction mixture. Not much change in the yield of total trans-
ester products was observed, when the rate of stirring was varied.
i.e. yield of total transester products were found to be independent
of the rate of stirring. This shows the absence of heat transfer limi-
tation and therefore it could be inferred that any external resistance
to its transfer from the bulk liquid phase to the external surface of
the catalyst is absent.
The mass transfer limitation can be evaluated by varying the
weight of the catalyst. A linear increase in the yield of transester
products with an increase in catalyst weight was observed (Fig. 4).
This indicates that the resistance to mass transfer is negligibly small
between the liquid bulk of reactant and the outer surface of the cat-
alyst particles. According to Madon and Boudart [30], which utilizes
the fact that in absence of all transport limitations, the rate of the
reaction is proportional to the weight of the catalyst.
3.2.3. Effect of reaction temperature
In order to study the kinetics and to optimize the reaction tem-
perature, transesterification of DEM with BA was carried out by
varying the reaction temperature between 353 and 453 K over zico-
nia, MZ and SZ catalysts. The obtained results are shown in Fig. 3.
The total transester yield and the selectivity towards DBM was
found to increase with reaction temperature upto 393 K indicating
that higher reaction temperature favour the formation of diester.
Temperature beyond 393 K, showed no considerable increase in the
percentage yield of total transester but instead the total transes-
ter yield decreased to a very small extent. This may be because of
the fact that when the reaction temperature was beyond 393 K, the
reactants or product molecules may undergo decomposition result-
ing in the formation of additional products other than BEM and
DBM. Colour change of reaction mixture to light brown at higher
temperature (>393 K) also imparts a light on this fact.
Therefore, the reaction temperature of 393 K was chosen as a
suitable reaction temperature for the optimization of other reaction
conditions.
Plots
of
−ln[1 − yieldtotal yield],
−ln[1 − yieldBEM
]
and
−ln[1 − yieldDBM] against reaction time at different tempera-
ture ranging from 353 to 413 K over ZrO2, MZ and SZ are shown in
Figs. 6–8 respectively.
3.2.4. Effect of weight of catalyst
The values of first-order rate constants obtained from the slopes
of these plots, the energy of activation calculated from the Arrhe-
nius equation (Eq. (1)) and the temperature coefficients (Eq. (2)) are
given in Table 3. The values of energy of activation of the catalyst for
transesterification were observed to be in the order: ZrO2 > MZ > SZ.
The energy of activation was calculated using Arrhenius equa-
tion [20]:
The effect of amount of catalyst on transesterification reaction
was studied in the range from 0.25 to 1.0 g. The obtained results are
presented in Fig. 4. The yield of total transester product increases
when weight of catalyst was increased from 0.25 g to 0.75 g and
a maximum yield was obtained at the catalyst weight at 0.75 g.
Further increase in catalyst weight showed not much increase in
the yield. So, 0.75 g of catalyst was chosen as catalytic amount for
further optimization of reaction condition.
ꢀ
ꢁꢂ
ꢃ
k2
k1
T1 × T2
T2 − T1
Ea = 2.303R log
(1)
(2)
3.2.5. Effect of molar ratio of the reactants
The molar ratio of DEM to BA (consisting of more BA) was varied
from 1:2 to 1:5. The obtained results are shown in Fig. 5a. The yield
of transester product increases when the molar ratio of DEM: BA
was increased from 1:2 to 1:3. Increase in the molar ratio beyond
1:3 showed a decrease in the yield of transester which may be due
to solvent effect of BA. It was also observed that an increase in the
concentration of BA increased the yield of DBM.
k2
Temperature coefficient =
k1
where, R, gas constant; k2, rate constant at temperature T2; k1, rate
constant at temperature T1.
Among all the catalysts SZ showed low energy of activation (Ea).
This indicates that, SZ is a suitable catalytic material for transesteri-
fication between DEM and BA with high yield of transester with
high selectivity towards DBM.
Similarly, molar ratio of BA to DEM (consisting of more DEM)
was varied from 1:0.33 to 1:2 and results are shown in Fig. 5b. It
was observed that not much change in the yield of total transester
yield was observed when the concentrationof DEM was increased.

N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
59
100
ZrO2
MZ
80
60
40
20
0
80
60
40
20
0
360
380
400
420
440
460
1
2
3
4
5
6
7
8
9
Temperature (K)
Reaction time
MZ
ZrO2
80
60
40
20
0
80
60
40
20
0
360
380
400
420
440
460
1
2
3
4
5
6
7
8
9
Temperature (K)
Reaction time (h)
80
60
40
20
0
SZ
SZ
80
60
40
20
0
360
380
400
420
440
460
1
2
3
4
5
6
7
8
9
Temperature (K)
Reaction time (h)
Fig. 3. Effect of reaction temperature. [Reaction conditions: reaction time = 6 h,
weight of catalyst = 0.5 g, molar ratio of DEM: BA = 1:3.] , Yield of total transester;
ꢀ, selectivity of BEM; and ꢁ, selectivity of DBM.
Fig. 2. Effect of reaction time. [Reaction conditions: reaction temperature = 433 K,
weight of catalyst = 0.5 g, molar ratio of DEM: BA = 1:3.] , Total transester yield; ꢀ,
selectivity of BEM; and ꢁ, selectivity of DBM.

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N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
100
ZrO2
ZrO2
80
60
40
20
0
80
60
40
20
0
0.2
0.4
0.6
0.8
1.0
1:2
1:3
1:4
1:5
Catalyst weight (g)
Molar ratio (DEM:BA)
MZ
MZ
80
80
60
40
20
0
60
40
20
0
0.2
0.4
0.6
0.8
1.0
0.2
0.4
0.6
0.8
1.0
Catalyst weight (g)
Molar ratio (DEM:BA)
SZ
SZ
80
60
40
20
0
80
60
40
20
0
0.2
0.4
0.6
0.8
1.0
1:2
1:3
1:4
1:5
Catalyst weight (g)
Molar ratio (DEM:BA)
Fig. 4. Effect of catalyst weight. [Reaction conditions: reaction time = 6 h, reaction
temperature = 393 K, molar ratio of DEM: BA = 1:3.]
selectivity of BEM; and ꢁ, selectivity of DBM.
, Total transester yield; ꢀ,
Fig. 5a. Effect of molar ratio (with excess of BA). [Reaction conditions: reaction
time = 6 h, reaction temperature = 393 K, weight of catalyst = 0.75 g.] , Total trans-
ester yield; ꢀ, selectivity of BEM; and ꢁ, selectivity of DBM.

N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
61
100
80
60
40
20
0
(ZrO2)
ZrO2
353 K
0.4
373 K
383 K
393 K
0.2
0.0
0
1
2
3
4
5
6
0.33:1
0.66:1
1:1
2:1
2:1
2:1
Time (h)
Molar ratio (DEM:BA)
1.0
0.5
0.0
(MZ)
MZ
90
353 K
80
70
60
50
40
30
20
10
0
373 K
383 K
393 K
0
1
2
3
4
5
6
0.33:1
0.66:1
1:1
Time (h)
Molar ratio (DEM:BA)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
SZ
(SZ)
353 K
373 K
383 K
393 K
80
60
40
20
0
0
1
2
3
4
5
6
0.33:1
0.66:1
1:1
Time (h)
Molar ratio (DEM:BA)
Fig. 6. Plot of first-order equation for the transesterification of DEM with BA over
ZrO2, SZ and MZ at 353 K, 373 K, 383 K and 393 K.
Fig. 5b. Effect of molar ratio (with excess of DEM). [Reaction conditions: reac-
tion time = 6 h, reaction temperature = 393 K, weight of catalyst = 0.75 g.]
, Total
transester yield; ꢀ, selectivity of BEM; and ꢁ, selectivity of DBM.

62
N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
(ZrO2)
0.35
353 K
0.30
373 K
383 K
393 K
0.25
0.20
0.15
0.10
0.05
0.00
(ZrO2)
353 K
373 K
383 K
393 K
0
1
2
3
4
5
6
1
2
3
4
5
6
Time (h)
Time (h)
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
0.35
0.30
0.25
0.20
0.15
0.10
0.05
(MZ)
353 K
373 K
383 K
393 K
(MZ)
353 K
373 K
383 K
393 K
0
1
2
3
4
5
6
1
2
3
4
5
6
Time (h)
Time (h)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
(SZ)
1.2
1.0
0.8
0.6
0.4
0.2
353 K
373 K
383 K
393 K
(SZ)
353 K
373 K
383 K
393 K
0
1
2
3
4
5
6
1
2
3
4
5
6
Time (h)
Time (h)
Fig. 8. Plot of first-order equation for the formation of DBM over ZrO2, SZ and MZ
at 353 K, 373 K, 383 K and 393 K.
Fig. 7. Plot of first-order equation for the BEM formation over ZrO2, SZ and MZ at
353 K, 373 K, 383 K and 393 K.

N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
63
80
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
ZrO2
(ZrO2)
60
40
20
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.0
2.0
2.2
0
1
2
3
4
5
Concn. of DEM (mol) x10-2
No. of reaction cycles
0.28
0.26
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
(MZ)
MZ
60
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.2
Concn. of DEM (mol) x10-2
1
2
3
4
5
No. of reaction cycles
(SZ)
0.6
0.5
0.4
0.3
0.2
SZ
80
60
40
20
0
0.0
0.5
1.0
1.5
Concn. of DEM (mol) x10-2
1
2
3
4
5
No. of reaction cycles
Fig. 9. Effect of DEM concentration on the initial reaction rate. [Reaction tempera-
ture = 393 K, catalyst weight = 0.75 g.]
Fig. 10. Reusability of catalyst. [Reaction conditions, reaction temperature = 393 K,
reaction time = 6 h, weight of catalyst = 0.75 g, DEM = 3.3 ml: BA = 6.7 ml (1:3).]
,
Total transester yield; ꢀ, selectivity of BEM; and ꢁ, selectivity of DBM.

64
N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
+ O
O
O
O
O
O
C
OH
O
OH+
O
O
O
O
O
OH
C6H5
O+
H
C6H5
C6H5
O+
H+
O
H+
O
C6H5CH2OH
O
O
O
O
O
H
OH
O
OH
O
OH
O
O
O
O
O
O
O
O
-C2H5OH
Similaraly
O
O
O
O
OH
OH
Scheme 2. Mechanism of transesterification reaction in presence of an acid catalyst.
Table 3
Reaction rate constant, energy of activation and temperature coefficients of ZrO2, MZ and SZ catalysts for transesterification of DEM with BA.
Catalyst
Rate constant ×10−3 minRate constant
Rate constant
Energy of activation (Ea) for
the formation of (kJ mol−1
Temperature
co-efficient (k2/k1)
(total transester)
×10−3 min (BEM)
×10−3 min (DBM)
)
383 K
393 K
383 K
393 K
383 K
393 K
Total yield
BEM
DBM
Total yield
BEM
DBM
ZrO2
MZ
SZ
0.97
1.67
3.85
1.39
2.17
4.49
4.16
3.30
1.56
3.57
2.56
1.60
0.64
0.83
4.00
0.95
1.04
4.44
45.10
32.78
19.20
−20.5
−31.8
49.44
28.23
13.06
1.43
1.30
1.17
0.86
0.78
1.03
1.48
1.25
1.11
3.17
3.4. Mechanism of the reaction
Fig. 9 indicates that the initial reaction rate increases linearly
with DEM concentration. This would suggest that the transesteri-
fication of DEM with BA follows an ER mechanism. Based on the
above assumptions a possible mechanism for the transesterifica-
tion of adsorbed DEM and BA over solid acid catalysts is given
in Scheme 2, which is similar to the mechanism of acid catalyzed
Fischer esterification [33].
In general DEM reacts with BA in presence of an acid catalyst
to give DEM and BEM (Scheme 1). This acid catalyzed reaction is
an equilibrium chemical reaction. In order to propose a possible
mechanism for this reaction the following work was taken up.
Either a Langmuir-Hinshelwood (LH) model or an Eley-Rideal
(ER) model is applicable for heterogeneously catalyzed transes-
terification reaction. The kinetics of these two mechanisms does
differ. For example LH kinetics involves the reaction between DEM
adsorbed on the catalyst surface and BA which is adsorbed on the
adjacent site of the catalyst. On the other hand, if the reaction fol-
lows an ER mechanism, then only one of the reactant species (DEM
or BA) is bound on the catalyst surface and is converted to the
product.
It has been reported in literature [31] that an adsorbed alco-
hol (BA) does not react, only blocks the active sites of the catalyst.
Therefore, it is assumed that BA from the liquid phase reacts with
adsorbed DEM. Furthermore, a decrease in the rate was observed
with an increase in the concentration of BA, this clearly indicates
the poisoning of the active sites of the catalyst. On the other hand,
an increase in the concentration of DEM increased the yield of total
transester with negligible deactivation (poisoning of active sites)
of the catalyst. Thus, it is assumed that it is DEM which is adsorbing
on the active sites of the catalyst and not BA.
3.5. Effect of re-usability of catalysts on the transesterification
reaction
The catalyst after the reaction was filtered, washed with ace-
tone, dried at 393 K for 2 h, calcined at 823 K for 2 h and reused for
transesterification of DEM with BA. The obtained results are shown
in Fig. 10. It can seen from Fig. 10 that there is not much change
in total transester yield, selectivity towards either BEM and DBM
over ZrO2, MZ and SZ catalyst. From this study we can conclude that
ZrO2, MZ and SZ catalysts are efficient and that they can be reac-
tivated and reused upto 5 reaction cycles without any appreciable
loss in their catalytic activity.
4. Conclusion
From a series of examination, it was observed that SZ is an
excellent catalyst compared to other catalyst used for the trans-
esterification of DEM with BA. A correlation between the yield of
transester and total surface acidity of the catalytic material was
observed. The selectivity of the products i.e. either BEM or DBM
was found to be related to the strength of acidic sites on the cat-
alytic material. A triangular correlation between the surface acidity,
tetragonal phase of zirconia and the catalytic activity of zirconia
Since there is no mass transfer limitation on the catalyst surface
as mentioned already, the chemical step was assumed to be the rate
limiting step. If the reaction proceeds through a LH mechanism,
then a plot of rate versus concentration of the reactants must pass
through a maximum, according to Narayanan and co-workers [32],
while if it follows an ER mechanism, no such maximum should
encountered.

N. Thimmaraju et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 55–65
65
catalysts was observed. All the catalysts used in the present study
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Acknowledgements
The authors acknowledge DST, New Delhi for project fund-
ing; Department of Chemistry, St. Joseph’s College, Bangalore for
PXRD analysis; Department of Chemical Engineering, University of
Malaya, Kuala Lumpur for surface area and surface acidity analy-
sis; Department of Organic Chemistry, IISc, Bangalore for GC–MS
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