Surface and catalytic properties of triflic acid supported zirconia: Effect of zirconia tetragonal phase

Article abstract of DOI:10.1016/j.molcata.2015.10.022

In this paper, trifluoromethanesulfonic acid was successfully loaded over calcined crystalline zirconia. Zirconia was calcined at 200, 350, 500, 700 and 900 °C before trifluoromethanesulfonic acid was loaded. The catalysts were characterized by various sophisticated techniques such as FTIR, SEM, TEM, XRD and N₂ adsorption-desorption measurements. The acidic behavior of the catalysts has been studied by nonaqueous potentiometric titration and FTIR spectra of adsorbed pyridine. The results showed that the catalysts are highly acidic and exhibited high catalytic activity in the esterification of acetic acid with benzyl alcohol (95%) with very high selectivity to benzyl ester (96%). Furthermore, the tetragonal phase of zirconia was the only phase up to 500 °C, and completely vanished at 900 °C. These changes in crystalline structure was also found to be related to the number and strength of acid sites. Our results showed that, the number and strength of acid sites were optimum at 350 °C, at which only tetragonal phase is the metastable one. Moreover, the catalysts were capable of recycling for five times without appreciable deactivation at 100 °C.

Full text of DOI:10.1016/j.molcata.2015.10.022

Accepted Manuscript
Title: Surface and catalytic properties of triflic acid supported
zirconia: Effect of zirconia tetragonal phase
Author: Hatem M. Altass Abd El Rahman S. Khder
PII:
S1381-1169(15)30125-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.10.022
MOLCAA 9665
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
4-10-2015
24-10-2015
26-10-2015
Please cite this article as: Hatem M.Altass, Abd El Rahman S.Khder,
Surface and catalytic properties of triflic acid supported zirconia: Effect
of zirconia tetragonal phase, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.10.022
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1
Surface and catalytic properties of triflic acid supported zirconia: Effect of
zirconia tetragonal phase
Hatem M. Altassa, Abd El Rahman S. Khdera,b* askhder2244@yahoo.com
aChemistry Department, Faculty of Applied Science, Umm Al Qura University,
Makkah, Saudi Arabia
bChemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt
*Corresponding author. Tel.: +966541472436.

2
Graphical abstract

3
Highlights




Pure Zirconia loaded with triflic acid was successfully prepared
The acidity was found to depend on the tetragonal phase of zirconia
The catalyst showed high activity (95%) with (97%) benzyl acetate selectivity
The catalyst is easily recoverable and reusable without much change in activity

4
Abstract
In this paper, trifluoromethanesulfonic acid was successfully loaded over
o
calcined crystalline zirconia. Zirconia was calcined at 200, 350, 500, 700 and 900 C
before trifluoromethanesulfonic acid was loaded. The catalysts were characterized by
various sophisticated techniques such as FTIR, SEM, TEM, XRD and N2 adsorption–
desorption measurements. The acidic behavior of the catalysts has been studied by
nonaqueous potentiometric titration and FTIR spectra of adsorbed pyridine. The
results showed that the catalysts are highly acidic and exhibited high catalytic activity
in the esterification of acetic acid with benzyl alcohol (95%) with very high
selectivity to benzyl ester (96 %). Furthermore, the tetragonal phase of zirconia was
o
o
the only phase up to 500 C, and completely vanished at 900 C. These changes in
crystalline structure was also found to be related to the number and strength of acid
sites. Our results showed that, the number and strength of acid sites were optimum at
o
350 C, at which only tetragonal phase is the metastable one. Moreover, the catalysts
were capable of recycling for five times without appreciable deactivation at 100°C.
Keywords: Trifluoromethanesulfonic acid; Zirconia; Tetragonal phase; Esterification
reaction; Benzyl alcohol;

5
1. Introduction
Acid catalyzed reactions are finding numerous applications in many areas of
the chemical industry. These are extremely useful in many large volume applications,
especially in fine chemicals manufacture and petroleum industry for alkylation and
isomerization reactions [1–4]. At the same point, solid acids are considered to be
more safe and eco-friendly than liquid acids such as, H2SO4, HCl, HF, AlCl3, BF3,
ZnCl2 and SbF5 because of their specific features such as non-corrosive, non-toxic,
easy to handle, easily recoverable, reusable and less expensive.
Metal oxides, in their supported or unsupported form, have been successfully
used as catalysts for many acid catalyzed organic transformations such as dehydration
[5], isomerization [6], acylation [7], oxidation [8], esterification reaction [9, 10],
Nitration [11], etc. Now-a-days metal oxide-based catalysts are active over a wide
range of temperatures and having high resistance to thermal excursions. Moreover,
the preparation methods, characterization tools and advantages of zirconia based solid
acids have gained much attention in various organic transformations due to their non-
toxicity, super acidity, simplified product isolation, high selectivity, reduction of
wasteful byproducts and high activity at low temperatures [12–14].
The trifluoromethanesulfonic acid (Triflic acid, CF3SO3H) has a highly acidic
nature and excellent thermal stability; it also has good resistance to reductive and
oxidative dissociation, with no generation of fluoride ions. The using of triflic acid in
heterogeneous catalysis by immobilization on a support is an interesting alternative to
contribute to the field of clean processes. Actually, There are few works on this
subject, though it can be mentioned its immobilization on some supports [15, 17].
The esterification of carboxylic acids and alcohols is fundamental reaction in
organic Synthesis [18,19]. Esters, which include a wide category of organic
compounds ranging from aliphatic to aromatic, are generally used in the chemical
industry such as drugs, pharmaceuticals, food preservations, perfumes, solvents and
cosmetics, [20,21]. Benzyl acetate finds numerous uses in perfumery, chemical
industries, and food [22].
In order to perform a new contribution to the field of ecofriendly acid-
catalyzed esterification reactions, triflic acid was immobilized on crystalline zirconia
o
obtained by sol-gel method. The effect of the calcination temperature (200-900 C)

6
and crystalline structure of the support on the physicochemical, acidic and textural
characteristics of the catalysts was studied. The catalytic behavior of the prepared
materials in benzyl acetate synthesis was studied through the esterification of acetic
acid with benzyl alcohol in a round flask glass reactor at atmospheric pressure. The
results are correlated with the catalyst properties.
Experimental
2.1. Synthesis of catalysts
2.1.1. Synthesis of zirconia
Zirconium isopropoxide (Sigma-Aldrich, 25 g) was mixed with isopropanol (Sigma-
Aldrich, 200 g) and stirred for 30 min to obtain a homogeneous clear solution under
N2 at room temperature. Then drops of 0.2 M NH4OH aqueous solution were dropped
slowly into the above mixture under vigorous stirring to catalyze the sol–gel reaction
for 2 h. After that, the mixture was left under sonication at room temperature for 2 h,
then the zirconia gel left to dry under vacuum overnight at room temperature. Finally,
and after complete dryness of the sample it was calcined at 200, 350, 500, 700 and
900°C for 4 h (Zr200, Zr350, Zr500, Zr700 and Zr900 oC, respectively).
2.1.1. Synthesis of zirconia supported Triflic acid
In three neck 50 ml round flask 2 g of precalcined zirconia was added to 20 ml dry
toluene under N2 atmosphere, the mixture was heated to reflux. Triflic acid,
CF3SO3H, (1.5 ml, Sigma-Aldrich, 98%) was slowly injected drop wise to the mixture
o
under vigorous stirring at 90 C under nitrogen atmosphere, then the mixture was
further refluxed under nitrogen atmosphere for 3 h. finally, the mixture was cooled,
filtered, washed with acetone and dried at 100ºC for 24 h under vacuum. The samples
were named TFA-ZrO2200, TFA-ZrO2350, TFA-ZrO2500, TFA-ZrO2700 and TFA-
ZrO2900.
2.2. Support and catalyst characterization
2.2.1. FTIR Spectra

7
FT-IR spectra of all calcined samples were recorded by using a Nicolet-Nexus
670 FTIR spectrophotometer (4 cm-1 resolution and 32 scans) in dried KBr (Sigma)
pellets and a measuring range of 400–4000 cm-1.
2.2.2. X-ray diffraction
X-ray powder diffraction patterns of samples were determined using an X’Pert
Philips Materials Research Diffractometer. The patterns were run with copper
radiation (Cu Kα, λ = 1.5405 Å) with the second monochromator at 45 kV and 40 mA
with a scanning speed of 2° in 2θ/min.
2.2.3. Textural properties
The adsorption isotherms and the specific surface area (SBET) of the various
catalysts were determined from nitrogen adsorption studies conducted at -196 °C
using a Quantachrome Autosorb-1-C system. The samples were previously degassed
at 100 °C for 2 h.
2.2.4. TEM and SEM images
Transmission electron microscope (TEM) images and the particle size were
obtained using a Jeol JEM-1230 operated at 120 kV. For TEM images the sample
powder was dispersed in methanol by using ultrasonic radiation for 10 min, and a
drop of the suspension was placed onto the carbon-coated copper grids.
Scanning electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDS) were carried out using a Quantum DS-130S Dual Stage Electron
Microscope.
2.2.5.Acidity measurements
2.2.5.1. Potentiometric titration
The total acidity of the solid samples was measured by means of
potentiometric titration [23-25]. The solid (0.05 g) was suspended in acetonitrile
(Merck), and agitated for 3 h. The suspension was then titrated with 0.1N n-
butylamine (Merck) in acetonitrile at 0.05 ml/min. The electrode potential variation
was measured with an Orion 420 digital A model by using a double-junction
electrode.

8
2.2.5.2. FTIR spectra of chemisorbed pyridine
Lewis and Brønsted acid sites present on the surface of the catalyst were
determined by FT-IR spectra of adsorbed pyridine. Prior to the pyridine adsorption
o
[23], small portions of the calcined samples were degassed under vacuum at 100 C
for 3 h, followed by suspending in dry pyridine. The excess pyridine was removed by
evaporation. The FT-IR spectra of the pyridine-adsorbed samples were obtained using
a Nicolet-Nexus 670 FTIR spectrophotometer by mixing 0.005 g of the sample with
0.100 g KBr in a resolution of 4 cm−1, and in a frequency range of 1400–1700 cm−1.
The ratio of Brønsted to Lewis acidity was determined based on the method in the
literature [26].
2.2.6. Catalytic activity
The esterification of Acetic acid with benzyl alcohol was carried out using 0.1
g of the catalyst. In a typical run, a mixture of 1 mmol of acetic acid (Aldrich, 99.9%)
and 1 mmol of benzyl alcohol (Sigma-Aldrich, 99.8%) were magnetically stirred and
heated to attain the reaction temperature (100 oC) in a round bottomed glass flask (50
ml) fitted with a water cooled condenser . Then activated catalyst (0.1 g) was added
and the mixture stirred for 2 h. The reaction mixture was then filtered and the
products were analyzed by means of GC-MS (HP GCD system equipped with FID
detector). The main product was found to be benzyl acetate with minor participation
of dibenzyl ether. More experiments were designed by varying the molar ratios of the
reactants, the reaction temperature and the reaction time to obtain various kinetic
parameters. To check the catalyst reproducibility, all the experiment were repeated
two times at the same reaction conditions, and each experiment was analyzed 3 times
using GC-MS equipment. The uncertainty was found to be ± 3% in most cases. The
yield and selectivity (in percentages) were calculated based on the GC analysis using
the following expressions:
ꢆꢇ ꢓ
ꢏ
100 ꢈꢂꢉꢊꢋꢃ ꢌꢍꢂꢎꢌꢎꢂ ꢐ ꢇꢄꢁꢈꢂꢉꢊꢋꢃ ꢂꢎꢑꢂꢒꢏ
ꢀꢁꢂꢃꢄ % ꢅ
ꢆꢇ ꢓ
ꢏ
ꢇ
ꢏ
ꢈꢂꢉꢊꢋꢃ ꢌꢍꢂꢎꢌꢎꢂ ꢐ ꢄꢁꢈꢂꢉꢊꢋꢃ ꢂꢎꢑꢂꢒ ꢐ ꢇꢌꢍꢂꢎꢁꢍ ꢌꢍꢁꢄꢏ
ꢆꢇ
ꢏꢓ
ꢏꢓ
ꢈꢂꢉꢊꢋꢃ ꢌꢍꢂꢎꢌꢎꢂ ꢐ ꢄꢁꢈꢂꢉꢊꢋꢃ ꢂꢎꢑꢂꢒ
100 ꢈꢂꢉꢊꢋꢃ ꢌꢍꢂꢎꢌꢎꢂ
ꢔꢂꢉꢊꢋꢃ ꢌꢍꢂꢎꢌꢎꢂ % ꢅ
ꢆꢇ
ꢏ
ꢇ

9
3. Results and discussion
3.1. FTIR spectra
The infrared spectra of ZrO2 200, TFA-ZrO2 200, TFA-ZrO2 350 and TFA-
ZrO2 500 are shown in Fig. 1. Inspection of Fig.1 reveals the following: (i) the
spectrum of the ZrO2 200 displays strong and broad band between the region of 3700–
3100 cm−1 centered at around 3450 cm-1 and another band at 1627 cm-1, assigned to
the stretching vibrations of hydroxo- and aquo-OH, and to the bending vibration of
(H–O–H) and (O–H–O) present in the structure of ZrO2 200 sample, respectively [27–
29].
(ii) moreover, the weak unresolved band between 800–520 cm−1 is
attributed to Zr–O stretching modes, FT-IR spectra of other unsupported zirconia
samples calcined at different temperatures showed features similar to those observed
for the ZrO2 200 with different intensities, (iii) the FTIR spectrum of zirconia
supported with triflic acid show additional bands, which are absent in pure
unsupported zirconia, these at 1265, 1180, and 1033 cm−1. The first two bands are
due to the S=O stretching mode of the adsorbed triflic acid on Zirconia support, and
the other sharp band at 1033 cm−1 is assigned to the C–F stretching [30,31].
These bands are also present in the spectra of all catalysts supported with
triflic acid, although with different intensities depending on the amount of triflic acid
adsorbed on the support. Thus, all these results of FTIR spectra indicate that, the
triflic acid was incorporated into the surface and inside the pores of zirconia.
3.2. Surface area measurement
Alongside, with the loading, incorporation or anchoring of any acid, base ,
metal or metal oxide in the framework position and/or into the walls of supporting
medium, there will be a progressive change in surface area, calcination temperature
was found to be the important reason of decreasing the surface area. The computed
values of the specific surface area (SBET) of the supports, together with the average
pore diameter (Dp) obtained from the BJH pore size distribution cited in Table 1. The
specific surface area (SBET) of pure zirconia was significantly decreased as the
calcination temperature increased from 200 to 900 oC. The specific surface area (SBET
)
was 347.6 m2/g for pure zirconia calcined at 200 oC and decreased to 63.8 m2/g when
o
the calcination temperature became 900 C. on the other hand, the average pore
diameter (Dp) of the samples slightly changed as the calcination temperature

10
increased but still in the mesoporous range (Table 1). On the other hand, the textural
properties of the zirconia calcined at different calcination temperature supported with
triflic acid were almost the same as those of the pure supports, thus indicating that
they are not altered by triflic acid addition. Similar results were observed when triflic
acid supported on titanium dioxide [16].
3.3. X-Ray diffraction
The effect of calcination temperatures on crystalline structure of prepared
zirconium oxide was studied using X-ray diffraction technique. Fig. 2 shows X-ray
diffraction profiles of ZrO2 calcined at different calcination temperatures.
Examination of Fig. 2 shows the following: (i) the X-ray pattern of pure zirconia
o
calcined at 200 C reveals a well-crystallized tetragonal phase of zirconia, (ii) the
tetragonal zirconia phase line located at 2θ = 30. 5◦ with crystal size of 11.42 nm, (iii)
o
the metastable tetragonal zirconia phase is thermally stable upto almost 500 C, (iv)
the diffraction lines of monoclinic phase start to appear when the calcination
o
temperature was further increased at 700 and 900 C at 2θ = 28.16◦, 31.44◦, (v) at
o
900 C the monoclinic zirconia phase becomes the predominant phase with a phase
ratio of 85.4% (Table 1).
o
When the calcination temperature increase up to 900 C the crystalline
structure of zirconia changes from tetragonal to monoclinic phase that was
accompanied with crystal growth and crystal size increase from 11.42 nm at 200 oC to
23.48 at 900 oC (Fig.2 and Table 1). The XRD patterns of the catalysts (TFA-ZrO2
200, TFA-ZrO2 350, TFA-ZrO2500, TFA-ZrO2 700 and TFA-ZrO2 900) showed
features similar to those of the corresponding supports used for their preparation (Fig.
1S supporting informations). This fact indicates that the impregnation of hydrated
zirconia with triflic acid did not appreciably affect the crystalline characteristics. On
the hand, the results showed that the change of the crystalline structure of zirconia
depended on calcination temperature.
The XRD results show that, the increase of the calcination temperature leads
to crystallite growth due to loss of H2O which leads to instability of tetragonal phase
[32]. Once the crystallites reach a critical size, the monoclinic phase becomes more
stable. The crystallite size of tetragonal phase changed from 11.42 to 13.27 nm when
o
the calcination temperature increased from 200 to 500 C (Table 1). During this
temperature and crystal size ranges the tetragonal phase of zirconia was more stable.

11
o
When the calcination temperature increased to 700 C, the crystal size increased by
more than 57 % (from 13.27 to 20.87 nm, Table1). This crystallite growth led to
instability of tetragonal phase which is due to sintering of zirconia particles due to the
elimination of H2O [33].
3.4. SEM and TEM images
SEM micrographs of ZrO2 200 and ZrO2 500 are shown in Fig. 3a, 3b . it can
be seen that the two zirconia samples had similar morphology. same spherical
particles were observed with almost similar size, Moreover, no clear effect of
calcination temperature on the morphology of the samples. The TEM images of ZrO2
200 and ZrO2 500 are also shown in Fig. 3c, 3d. The images of the zirconia samples
were completely correlated with XRD observations. The observation in both cases
(TEM and XRD) of a dominant (single-type) particle morphology supports the
monophasic nature of the catalysts. The XRD showed 100 % tetragonal crystal
o
structure which still thermally stable with 100 % at 500 C. The average particle size
measured from TEM images for the two zirconia samples were found to be in the
range of (7-11 nm). Moreover, TEM images of both catalysts also showed small dark
spots that could be due to Zr-rich domains.
3.5.Acidity measurements
Two techniques were applied to measure the acidic properties of the prepared
solids. The total number of acidic sites and their relative acidic strength were
measured using nonaqueous potentiometric titration of n-butylamine in acetonitrile.
On the other hand, the type of acid sites Lewis (L) and Brönsted (B)were also studied
using FTIR spectra of chemisorbed pyridine.
3.5.1. Nonaqueous potentiometric titration.
The acidity of all supported and unsupported zirconia catalysts were firstly
characterized by nonaqueous potentiometric titration with n-butylamine in
acetonitrile. Using this technique, it is possible to estimate the relative strength and
the total number of acid sites present in the catalysts. As a criterion to interpret the
results, it was suggested that the initial electrode potential (Ei) indicates the maximum
acid strength of the sites, and the value of meq amine/g catalyst where the plateau is

12
reached indicates the total number of acid sites [34]. On the other hand, the acid
strength of these sites may be classified according to the following scale: Ei >100 mV
(very strong sites), 0< Ei <100 mV (strong sites), -100< Ei <0 mV (weak sites) and
Ei<-100 mV (very weak sites) [34].
The titration curves of the triflic acid supported catalysts are shown in Fig. 4.
According to the above classification, the samples presented very strong acid sites,
with Ei values in the range 211–802 mV. The total number of acid sites determined by
potentiometric titration method decreased in parallel with the calcination temperature.
Additionally, the acid strength of the catalysts decreased in the following order TFA-
ZrO2200 (802 mV) > TFA-ZrO2350 (779 mV) > TFA-ZrO2500 (720 mV) TFA-
ZrO2700 (299 mV) > TFA-ZrO2900 (211 mV). Moreover, according to the acidity
values listed in Table 1 both the number and strength of acid sited of unsupported
zirconia also decreased in parallel with the calcination temperature, where , both of
them becomes minimum at 900 oC. Moreover, the Ei values of Pure zirconia samples
(Table 1 and Fig. 2S supporting information) were in the range 12-111 mV. Table
1 showed also that the acid strength and number of acid sites of the pure unsupported
ZrO2 samples decreased in parallel with the increase of calcination temperature. The
support acid strength and number of acid sites decrease as a result of the partial
dehydroxylation that takes place during the thermal treatment of the solids.
3.5.2. FTIR spectra of chemisorbed pyridine
More information for the surface acid-base properties of triflic acid supported
zirconia catalysts were obtained by FTIR measurements of adsorbed pyridine (Fig. 5).
As shown in Fig. 5 all the catalysts composed of mixture of Lewis (L) and Brönsted
(B) acid sites. The spectra of all samples consist of broad intensive bands with a
maximum at 1444, 1540 and 1488 cm−1, which according to [35] could be assigned to
Pyridine adsorbed on Lewis (L)acid sites, pyridine adsorbed on Brönsted (B) acid
sited, respectively. In addition, the band at 1488 cm-1 indicates the formation of the
adjacent Lewis (L) and Brönsted (B) acid sites.
The Brönsted/Lewis (B/L) sites ratio was calculated from the ratio of the peak
areas of the bands at 1540 and 1444 cm−1, respectively. The B/L ratios are depicted in
Table 1 as a function of calcination temperature. These results indicated that B/L ratio
decrease with the rise of calcination temperature upto 900 oC. this expected decrease
in B/L ratio may be due to the gradual loss of H2O molecules that accompanied the
rise of calcination temperature to 900 oC .

13
3.6. Catalytic activity
The catalytic activity of pure ZrO2 and triflic acid supported ZrO2 was
evaluated in the esterification reaction of acetic acid (AA) with benzyl alcohol (BA).
Screening of the reaction parameters including temperature, BA/AA molar ratio,
reaction time was made, catalyst reuse and the esterification reaction was also
performed in the absence of the catalyst at the same conditions.
3.6.1. Effect of triflic acid and calcination temperature
Blank experiment was firstly run under without catalyst, using 0.1 g of the
o
catalyst, BA/AA molar ratio 1:1 at 100 C for 2 h under vigorous stirring. The
percentage yield of the reaction was 4.6 % , and the yield was benzyl acetate with
100% selectivity. The uncertainty was found to be between 1~3% for the percentage
yield and selectivity, under these reaction conditions.
The effect of calcination temperature of the catalysts on the catalytic activity
towards BA esterification by AA at the same reaction conditions using 0.1g catalyst
was studied and the results are shown Fig. 6 a, b. As shown in Fig. 6a, when the
o
calcination temperature increased from 200 to 350 C, the %yield of the reaction is
increased by around 3%. Further increase in the calcination temperature from 350 to
o
900 C was accompanied with notable decrease in the % yield of the reaction.
Moreover, when pure zirconia samples were used in the reaction, the sample calcined
at 350 oC exhibited higher catalytic activity than the other samples (Fig. 6b).
The results show the correlation between catalytic activity and surface acidity.
The samples loaded with triflic acid exhibited much higher catalytic activities the pure
zirconia samples, which is due to loading of triflic acid on zirconia surface. Although,
o
the number of acid sites in the samples calcined at 200 C seems to be little higher
o
than the samples calcined at 350 C due to partial dehydroxylation, but the samples
calcined at 350 oC exhibited higher catalytic activity than the other samples. This may
o
o
be due to the strength of acid sites at 350 C are higher than 200 C. The results also
show the formation of dibenzyl ether as a minor product with benzyl acetate (major
product) only with the samples loaded with triflic acid. Nandiwale et al. [36] reported
that the formation of dibenzyl ether during the esterification reaction of benzyl
alcohol and acetic acid depends on the porous structure of the support, small or
medium pores cannot accommodate the large ether molecule (dibenzyl ether) and thus
hinders its formation. We think the formation of dibenzyl ether is related not only to

14
surface acidity but also to its strength, this may explain the formation of ether with
triflic acid supported samples.
3.6.2. Effect of reaction temperature
Further, the reaction was studied at 60, 80 and 100 oC, using 0.1 g of TFA-
ZrO2 350, BA/AA molar ratio 1:1, for 2h (Fig. 7), the uncertainty between 1~2 % .
The results showed that, the yield % was small at low temperatures, it was only 85 %
at 60 oC. The yield % increased with increase in temperature. Maximum yield of 94 %
o
was obtained at 100 C. Moreover, the dibenzyl ether selectivity was maximum at
o
o
60 C (12.3%) and decreased to minimum value (3.5%) at 100 C. Although,
Kirumakki et al.,[37] reported that, the selectivity towards dibenzyl ether increased on
o
increasing the reaction temperature from 110 to 130 C, but our results showed that
the formation of dibenzyl ether is favored at low reaction temperature, which may be
due to gradual decomposition of dibenzyl ether at higher reaction temperatures.
3.6.3. Effect of reaction time
o
The effect of reaction time was studied over 0.1 g TFA-ZrO2 350 at 100 C,
using BA/AA molar ratio 1:1, for under vigorous stirring (Fig. 8), the uncertainty
between 1~3 %. As it can be seen in Fig. 8, the percentage yield increased quickly
(55%) after 30 min, after 60 min. the % yield becomes 91% and becomes almost
constant after 120 min., at 180 min., no significant change in the yield % is observed,
which suggests that, 120 min. are enough for the reaction.
The catalyst filtration test proved heterogeneous nature of catalysis over TFA-
ZrO2 350 catalyst after fast removal of the catalyst after 30 min. the % yield was
almost 55% after 30 min. and after removal of the catalyst, the % yield still without
significant changes after 180 min. These results mean that the reaction in the filtrate
stopped completely, indicating that no active triflic acid species was leached into the
solution during the reaction.
3.6.4. Effect of molar ratio
The esterification reaction between acetic acid (AA) and benzyl alcohol (BA)
was carried out by varying the molar ratio of AA to BA from 1:1 to 1:4 over TFA-
ZrO2 350 sample to assess its effect on the catalytic activity and selectivity (Fig. 9),
the uncertainty between 1~2 %. The % yield was found to increase from 93 to 98 %
with an increase in reactant molar ratio from 1:1 to 1:2. Further increase in molar ratio

15
was accompanied with small decrease in the % yield, where the % yield become 90%
when the molar ratio of AA to BA become 1:4. When increase in the molar ratio of
AA to BA from 1:1 to 1:4, the selectivity of benzyl acetate was found to decrease
from 96% at molar ratio of 1:1 to 92% at molar ratio of 1:2, and become 94 % at
molar ratio 1:4. The results showed that the % yield was maximum (97.7 %) at molar
ratio of AA to BA of 1:2, while the selectivity of benzyl acetate was maximum (96%)
at molar ratio of AA to BA of 1:1. Thus, all the reactions in this work were performed
with a molar ratio of AA to BA was 1:1.
3.6.5. Catalyst reuse
Recycling of the catalyst is an important aspect of any industrial process. For
this purpose, reusability of the catalyst is tested by carrying out repeated runs of the
o
reaction over TFA-ZrO2 350 at 100 C, the reactants molar ratio (AA to BA) at 1:1,
using 0.1 g catalyst, the results are shown in Fig. 10, the uncertainty between 1~3 %.
As it can be seen in Fig. 10, after the first run the yield % was 93 % and slightly
decreased to 90 % after the fourth run, and decreased a little more (85 %) after the
fifth run. Moreover, the selectivity to benzyl acetate was almost constant (95~96 %)
even after the catalyst reused 5 times. These results concludes that, our catalyst is
highly active, stable and reusable. The present method for esterification of AA with
BA over triflic acid supported tetragonal zirconia catalyst offers greener methodology
with potential advantages with respect to higher yield %, high selectivity towards
benzyl acetate (95%) and catalyst reusability for five cycles without considerable loss
in activity.
According to XRD, surface acidity and catalytic activity results, there is a
relation between zirconia phase change and surface acidity. Aramendia et al. [38],
reported that, the phase transformation of zirconia is responsible for the change in the
strength distribution of the acid sites. In zirconia, the dehydroxylation process due to
o
thermal treatment up to 900 C was accompanied with changes in the crystalline
structure from pure tetragonal zirconia to mixture of tetragonal and monoclinic phases
and finely pure monoclinic phase. These changes in crystalline structure was also
found to be related to the number and strength of acid sites. Our results showed that,
o
the number and strength of acid sites were optimum at 350 C, at which tetragonal
phase is the metastable one. Moreover, many authors reported that the tetragonal
phase is the active phase [39,40] and the transformation from the metastable

16
tetragonal phase to the monoclinic phase is related to the deterioration of the acidity
o
of the samples [41, 42]. This may explain why the samples calcined at 350 C
exhibited higher catalytic activity compared to other samples calcined at other
calcination temperature.
4. Conclusion
Triflic acid was successfully supported over calcined zirconia, the zirconia
was calcined at 200, 350, 500, 700 and 900 oC. The catalysts have been characterized
by various sophisticated techniques such as XRD and N2 adsorption–desorption
measurements. The acidic behavior of the catalysts has been studied by nonaqueous
potentiometric titration and FTIR spectra of adsorbed pyridine. The effect of the triflic
acid loading and calcination temperature of zirconia on the textural characteristics of
the catalysts has also been studied and the results have been clearly discussed.
Esterification of BA with an AA was carried out over the catalysts. The catalysts
showed excellent catalytic activity towards the selected reaction (95 %) with very
high selectivity to benzyl ester (96 %) . Catalytic activities have been correlated with
the acidity of the catalysts. Effects of reaction parameters such as mole ratio of
reactants, reaction time and temperature, and reusability of the catalyst have been
studied so as to get higher substrate conversions and product selectivity. The above
catalyst was recyclable, cost effective and environmental friendly and could be used
in similar reactions.

17
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19
Figure Captions
H-O-H
O-H-O
OH
C-F
S=O
(a)
Zr-O
(b)
(c)
(d)
(e)
(f)
4000
3000
2000
1000
-1
Wavenumber (cm )
Fig. 1. FTIR spectra of: (a) ZrO2 (b) TFA-ZrO2200, (c) TFA-ZrO2350, (d) TFA-
ZrO2500, (e) TFA-ZrO2700 and (f) TFA- ZrO2900

20
M
M
T
T
T
(e)
(d)
(c)
(b)
(a)
20
30
40
50
60
70
80

Fig. 2. XRD diffraction patterns of ZrO2 samples calcined at (a) 200, (b) 350, (c) 500,
(d) 700 and (e) 900 oC

21
a
b
c
d
Fig.3. SEM images of (a) ZrO2 200, (b) ZrO2 500 and TEM images of (c) ZrO2 200,
(d) ZrO2 500

22
TFA-ZrO 200
2
800
600
400
200
0
TFA-ZrO 350
2
TFA-ZrO 500
2
TFA-ZrO 700
2
TFA-ZrO 900
2
-200
0.0
0.5
1.0
1.5
2.0
2.5
3.0
meq n-butylamine/g catalyst
Fig.4. Nonaqueous potentiometric titration curves of zirconia loaded with triflic acid
at different calcination temperature.

23
B/L
L
B
a
b
c
d
e
1600
1550
1500
1450
1400
-1
Wavenumber (cm )
Fig.5. FTIR spectra of chemisorbed pyridine over (a) TFA-ZrO2200 (b) TFA-
ZrO2350, (c) TFA-ZrO2500, (d) TFA-ZrO2700 and (e) TFA-ZrO2900

24
120
100
80
60
40
20
0
% Yield
% Benzlyacetate selectivity
(a)
100 200 300 400 500 600 700 800 900 1000
Calcination temperature, oC
100
(b)
80
60
40
20
0
100 200 300 400 500 600 700 800 900 1000
Calcination temperature, oC
Fig.6. Esterification of benzyl alcohol with acetic acid: Effect of calcination
temperature, using 0.1 g catalyst; at 100 oC; using AA/BA molar ration 1:1; for 2h on
(a) Triflic acid supported zirconia (b) zirconia, at different calcination temperature.

25
% Yield
% Benzyl ester selectivity
% di benzyl ether selectivity
100
80
60
40
20
0
50
60
70
80
90
100
110
Reaction temperature, oC
Fig.7. Esterification of benzyl alcohol with acetic acid: effect of reaction temperature
over TFA-ZrO2 350. using 0.1 g catalyst; AA/BA molar ratio 1:1; for 2h .

26
With catalyst
Without catalyst
100
75
50
25
0
catalyst filtration
0
30
60
90
120
150
180
Reaction time, min.
Fig.8. Esterification of benzyl alcohol with acetic acid: effect of reaction time over
TFA-ZrO2 350 . using 0.1 g catalyst; using AA/BA molar ration 1:1; at 100 oC

27
% Yield
% Benzyl ester
% dibenzyl ether
100
75
50
25
0
1:1
1:2
1:3
1:4
Molar ratio (AA/BA)
Fig.9. Esterification of benzyl alcohol with acetic acid: effect of reactants molar ratio
over TFA-ZrO2 350. using 0.1 g catalyst; for 2h; at 100 oC .

28
120
100
80
60
40
20
0
% Yield
% benzyl ester
% dibenzyl ether
Run 1
Run 2
Run 3
Run
Run 4
Run 5
Fig.10. Esterification of benzyl alcohol with acetic acid: effect of catalyst durability
over TFA-ZrO2 350. using 0.1 g catalyst; using AA/BA molar ration 1:1; for 2h; at
100 oC.

29
Tables
Table 1 Physicochemical properties of pure and triflic acid supported zirconia
catalysts
Sample
%
Tetragonal
phase
100.00
--
Crystal SBET
Dp
E(i)
B/L
mmol n butylamine / g
catalyst
size
(m2/g)
(nm)
ratio
(nm)
ZrO2 200
TriZrO2200
ZrO2 350
11.42 347.61
-- --
12.01 301.44
-- --
13.27 244.08
-- --
20.87 119.76
3.4
--
+111
+802
+100
+779
+61
--
1.89
--
0.6
1.80
0.60
1.68
0.35
0.83
0.12
0.21
0.08
0.12
100.00
--
3.4
--
TriZrO2350
ZrO2 500
1.78
--
100.00
--
3.5
--
TriZrO2500
ZrO2700
+720
+42
1.47
--
51.06
--
3.7
--
TriZrO2700
ZrO2 900
--
23.48
--
--
63.84
--
+299
+12
1.07
--
14.60
--
3.9
--
TriZrO2900
+211
0.51