Esterification of cetyl alcohol with palmitic acid over WO3/Zr-SBA-15 and Zr-SBA-15 catalysts

Article abstract of DOI:10.1016/j.apcata.2016.05.010

Tungsten loaded and Zr incorporated SBA-15 catalysts (WO₃/Zr-SBA-15 and Zr-SBA-15) were developed for esterification of cetyl alcohol and palmitic acid. The influence of the Zr content, tungsten loading amount, calcination temperature, feed composition and catalyst amount has been studied. Higher tungsten loading decreased the acidity due to formation of WO₃ crystals whereas calcination temperature enhanced the acidity by favoring the dispersion of WO_x species. Activities of the catalyst changed depending on their amount of Br?nsted sites and total number of acid sites. Zr-SBA-15 catalyst which had the highest amount of Br?nsted acid sites gave maximum cetyl palmitate yield (63.1%). This catalyst retained its activity up to 3 reuse cycles without significant loss of activity.

Full text of DOI:10.1016/j.apcata.2016.05.010

Applied Catalysis A: General 522 (2016) 194–200
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Feature article
Esterification of cetyl alcohol with palmitic acid over WO /Zr-SBA-15
3
and Zr-SBA-15 catalysts
∗
Vahide Nuran Mutlu, Selahattin Yilmaz
I˙ zmir Institute of Technology, Chemical Engineering Department, Gülbah c¸ e Kampüs, Urla I˙ zmir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Tungsten loaded and Zr incorporated SBA-15 catalysts (WO /Zr-SBA-15 and Zr-SBA-15) were developed
for esterification of cetyl alcohol and palmitic acid. The influence of the Zr content, tungsten loading
3
Received 15 February 2016
Received in revised form 6 May 2016
Accepted 10 May 2016
amount, calcination temperature, feed composition and catalyst amount has been studied. Higher tung-
sten loading decreased the acidity due to formation of WO3 crystals whereas calcination temperature
enhanced the acidity by favoring the dispersion of WOx species. Activities of the catalyst changed depend-
ing on their amount of Brønsted sites and total number of acid sites. Zr-SBA-15 catalyst which had the
highest amount of Brønsted acid sites gave maximum cetyl palmitate yield (63.1%). This catalyst retained
its activity up to 3 reuse cycles without significant loss of activity.
Available online 11 May 2016
Keywords:
Cetyl palmitate
Cetyl alcohol
©
2016 Elsevier B.V. All rights reserved.
Esterification
Mesoporous acid catalysts
Zr-SBA-15
Tungsten loaded
1
. Introduction
large amount of catalyst and feedstock. Therefore, it is required to
develop active, reusable and environmentally friendly heteroge-
neous catalysts.
Esters of fatty acids are used in many different areas, such
as oiling agents and emulsifiers in food industry, personal care
emollients surfactants and base materials for perfumes in cos-
metic industry, solvents, lubricants for plastics, paint and ink
additives. Among the different fatty acid esters, cetyl esters are
commonly applied in cosmetic industry. Cetyl palmitate, cetyl
stearate, myristyl myristate, and myristyl stearate are different
types of cetyl esters. In the present study cetyl palmitate was pro-
duced by esterification of cetyl alcohol and palmitic acid through
the reaction given below (Scheme 1).
Fatty acid esters are synthesized by esterification of fatty acids
and alcohols in the presence of an acid catalyst and the reaction
mechanism is called Fischer esterification. Fischer esterification
reaction proceeds by nucleophilic attack of the alcohol on the pro-
tonated carbonyl group of the carboxylic acid to form a tetrahedral
intermediate as shown in Scheme 2. Collapse of the intermediate
regenerates the carbonyl group and produces the ester and water.
Conventionally mineral acids such as concentrated sulfuric acid and
hydrochloric acid are used as catalysts. These catalysts cause prob-
lems such as environmental hazards, difficulty in catalyst recovery
and waste control, high susceptibility to water, requirement of a
Recent studies on heterogeneous catalysts for esterification
reactions include zeolites [1,2], acidic resins [3,4], sulfated zirco-
nia [5,6], tungstated zirconia [7–9] and heteropoly acids [10,11].
However, the success of the catalysis was limited to esters less
than C10 in either carboxylic acids or alcohols [12]. Sakthivel et al.
studied esterification of different fatty acids and fatty alcohols over
MCM-41 supported tungstophosphoric acid catalysts under super-
critical conditions and reported high ester yields. However, catalyst
leaching was observed [13]. Mantri et al. tested supported and
unsupported multivalent metal salt hydrates for the esterification
of long chain fatty acids and fatty alcohols. ZrO2+ was found to be an
active metal cation for the esterification reaction with some other
metals like Ga3+ and In . Moreover, mesitylene was reported as
a promising solvent with a yield of 89.3% at its boiling tempera-
3+
◦
ture (162 C) in the same study among different solvents including
benzene, toluene, m-xylene, diethylbenzene and tetralin [12].
Tungsten oxide based catalysts have been reported as active,
thermally stable and promising catalysts for acid catalyzed reac-
tions such as hydrocarbon isomerization [14–16], oxidation [17],
acetylation [18] etherification and alkylation [19]. Ramu et al. stud-
ied esterification of palmitic acid with methanol over WO -ZrO2
3
catalysts prepared by impregnation and co-precipitation methods
and obtained fatty acid conversions up to 95% [7]. Park et al. com-
pared WO -ZrO , sulfated ZrO and Amberlyst-15 for conversion of
∗ Corresponding author.
E-mail address: selahattinyilmaz@iyte.edu.tr (S. Yilmaz).
3
2
2
http://dx.doi.org/10.1016/j.apcata.2016.05.010
0
926-860X/© 2016 Elsevier B.V. All rights reserved.

V.N. Mutlu, S. Yilmaz / Applied Catalysis A: General 522 (2016) 194–200
195
Scheme 1. Cetyl alcohol esterification with palmitic acid.
used vegetable oils to fatty acid methyl esters, and WO -ZrO gave
as reaction temperature, solvent type, and reactant ratio, while the
effects of the catalyst preparation methods on the acidity and sur-
face properties of the catalysts and on the yield of the esters were
not examined in detail [12,13,32,33].
3
2
the highest conversion (93%) and no leaching of WO was observed
3
in the esterification reaction [8]. This was attributed to the strong
interactions between WO3 and ZrO2 and high acidity of the cata-
lysts. Despite their high activity and stability, WO -ZrO catalysts
In the present work, the use of Zr incorporated SBA-15 (Zr-SBA-
3
2
have the disadvantage of small surface area and non-uniform pore
size which may limit their activity for catalyzing the esterification
of long chain fatty acids and fatty alcohols.
15) and tungsten oxide impregnated WO /Zr-SBA-15 mesoporous
3
acidic catalysts in the esterification of cetyl alcohol by palmitic acid
was reported for the first time. Effects of zirconia and tungsten con-
tents, calcination temperature on the catalyst properties, activity
and ester yield were examined. Different catalyst amounts and feed
compositions were also studied.
Mesoporous silicas were declared as promising catalyst sup-
ports in different studies [13,20,21]. Among different mesoporous
silica materials SBA-15 was chosen for this study because of its
high surface area and much thicker walls and consequently higher
hydrothermal stability than MCM-41 [22,23]. To improve the sur-
face acidity and broaden the applications, different metal ions
have been incorporated into SBA-15 [24–27]. Garg et al. synthe-
sized SBA-15 containing various amount (10–50 wt%) of highly
dispersed ZrO2 by urea hydrolysis method and tested these cata-
lysts in esterification of cyclohexanol with acetic acid and cumene
cracking. It was observed that, Zr doping did not destroy the hexag-
onal structure of SBA-15. As the ZrO2 amount increased the total
acidity and Brønsted acidity of the catalysts increased whereas
the Lewis acidity decreased [28]. Biswas et al. prepared Zr-SBA-
2. Experimental
2.1. Catalysts preparation
Zr incorporated SBA-15 catalysts were prepared according
to the method published by Zhang et al. with two dif-
ferent Zr molar amounts of 0.017P123:Si:0.08Zr:220H O and
2
0
.017P123:Si:0.1Zr:220H O). The catalysts were named as Zr-
2
SBA15-08 and Zr-SBA15-10, respectively. ZrOCl ·8H O was used
2
2
as Zr precursor and tetraethyl orthosilicate (TEOS) was used as sil-
ica source. The synthesis was carried out with the preparation of a
clear aqueous micelle solution of amphiphilic triblock copolymer
P123. Then, ZrOCl ·8H O solution with determined concentration
1
5 by chemical grafting (post synthesis) and direct hydrothermal
synthesis. Zr-SBA-15 with higher surface area was obtained by
direct hydrothermal synthesis. It was also reported that Zr loading
improved the acidity of the catalyst [29]. Gracia et al. also investi-
gated the hydrothermal synthesis of Zr-SBA-15. They synthesized
well-structured Zr-SBA-15 mesoporous materials with high surface
areas and narrow pore size distributions, containing both Lewis and
Brønsted acid sites [30].
2
2
and tetraethyl orthosilicate (TEOS) was added into the micelle solu-
◦
tion and stirred at 37 C for 24 h. Afterwards, the gel was placed in
◦
an autoclave and heated to 90 C for another 24 h without stirring.
The solid product was recovered by centrifuge, washed with de-
ionized water and dried overnight at ambient conditions. The dried
The effect of tungsten loading on Zr-SBA-15 was studied by
Zhang et al. for the benzoylation of anisole. A series of strong acids
◦
Zr-SBA-15 was calcined at 800 C for 4 h in air flow.
Tungsten was loaded on to Zr-SBA-15 (15 wt% and 20 wt%)
by incipient wetness impregnation from aqueous ammonium
metatungstate solutions. The catalysts were kept in an ultrasonic
WO /Zr-SBA-15 were prepared and the materials retained their
3
mesoporous structure after WO3 loading. The catalysts possessed
both Lewis and Brønsted acid sites and exhibited considerably high
activity for the benzoylation of anisole [31].
The studies about the esterification of long chain fatty acids and
alcohols in the literature investigated the reaction parameters such
bath in order to improve WO dispersion over Zr-SBA-15 supports.
3
◦
After filtration, the product was dried at 110 C for 12 h, and then
the powder obtained was heated to calcination temperature with a
◦
◦
heating rate of 2 C/min. The catalysts were calcined at 700 C and
Scheme 2. Fischer esterification mechanism.

1
96
V.N. Mutlu, S. Yilmaz / Applied Catalysis A: General 522 (2016) 194–200
◦
00 C for 3 h and named as follows. For example, catalyst prepared
Table 1
8
◦
Influence of stirring rate on the initial disappearance rate and conversion of cetyl
alcohol and the yield of cetyl palmitate over Zr-SBA15-08.
with initial 0.08 mol of Zr, loaded 15% WO and calcined at 700 C
3
was referred to as WZSBA08-15-7.
Stirring
rate (rpm)
r0× 105
(mol min
Conversion
(%)
Yield
(%)
−
1
−1
a
a
g
)
2
.2. Characterization
5
7
20
50
9.23
9.59
64.3
64.6
63.1
63.8
Surface properties of catalysts were analyzed by nitrogen
a
adsorption-desorption using Micromeritics ASAP 2010. N adsorp-
At 6 h reaction time.
2
◦
tion was performed at 77 K following the degas step at 200 C for
2
h. Specific surface areas were calculated by the BET method (SBET).
Structural analysis of the catalysts were performed by X-ray
◦
◦
temperature was 280 C and the detector temperature was 320 C.
◦
◦
The GC oven temperature was changed from 50 C at 12 C/min to
00 C where it was kept for 35 min. Helium was used as the car-
rier gas at a flow rate of 37.3 ml/min The split ratio was 24.9:1.
Conversion of cetyl alcohol (CA), yield of cetyl palmitate (CP) and
the selectivity to CP were defined as below.
diffraction and Raman spectra. The XRD patterns were obtained
by Philips X’Pert diffractometer with CuK␣ radiation. The scatter-
ing angle 2␪ was varied from 5 to 80, with a step length of 0.02.
Raman spectra of the catalysts was obtained by an Argon laser at
◦
3
−
1
the excitation wavelength of 488 nm. The resolution was 4 cm
.
Conversion(%) = (
CAin − CAout)
× 100
The framework vibration of synthesized Zr-SBA-15 and WO /Zr-
3
SBA-15 catalysts were examined by FTIR spectroscopy. KBr pellet
technique was employed to obtain infrared spectra of the samples
at room temperature. The pellets were prepared with a sample
amount of 3 wt%. The spectra was retrieved in the wavenumber
range of 400–2000 cm with a resolution of 4 cm by an infrared
spectrometer type Schimadzu FTIR 8400S.
CAin
CPout
Yield(%) =
× 100
CAin
−1
−1
CPout
CAin − CAout)
Selectivity to CP(%) =
× 100
NH -TPD method was applied to determine acid site distri-
3
(
bution and quantify acidity of the catalysts using Micromeritics
AutoChem II Chemisorption Analyzer. In a typical analysis, the pow-
der was heated to 400 C with a heating rate of 5 C/min and kept
at 400 C for 30 min under 70 ml/min He gas flow. After cooling to
6
The reusability of the three catalysts providing the most ester
yields were tested. For this, the used catalysts were separated
from the reaction mixture by filtration and washed twice with
mesitylene and methanol. Subsequently, the catalysts were dried
overnight at ambient conditions and heat treated at 550 C for 6 h.
Then, they were tested in the reaction again.
◦
◦
◦
◦
◦
0 C with a rate of 5 C/min and reducing He flow to 30 ml/min, it
◦
was subjected to NH -He flow at the rate of 30 ml/min for 30 min.
This was followed by degassing step at 60 C under He flow of
3
◦
7
0 ml/min for 2 h in order to remove the physically adsorbed NH .
3
◦
TCD signals were taken as the sample was heated to 700 C with a
rate of 10 C/min.
3. Results and discussion
◦
Brønsted and Lewis acidity characteristics of the catalysts
were determined by IR spectroscopy using pyridine adsorp-
tion/desorption method. For the analysis, the samples were kept
3.1. Catalyst characterization
3.1.1. Phase characterization and surface properties
◦
−2
at 300 C under vacuum (2 × 10 mm Hg) for 2 h. Pyridine adsorp-
The XRD analysis showed that the hexagonal structure of SBA-15
was preserved after Zr incorporation (results not reported). When
the spectra of Zr-SBA15-08 and Zr-SBA15-10 were compared, it was
observed that the intensities of the characteristic peaks decreased
with increasing Zr amount. This might be attributed to the decay
in the orderliness of the structure because of the presence of
more Zr in the mesoporous channels. Fig. 1 displays the XRD pat-
◦
tion was performed at 150 C for 30 min. Then, the sample was kept
◦
at 120 C for 2 h under N flow of 30 ml/min for desorption of the
2
physisorbed pyridine. Shimadzu FTIR 8400S model Fourier Trans-
formed Infrared Spectrometer was used to carry out the IR analysis
−
1
of sample pellets between 400 and 4000 cm . The sample pel-
lets were prepared using KBr. The concentration of the pyridine
adsorbed catalyst in the KBr pellet was 3 wt%.
terns of WO /Zr-SBA-15 catalysts. The labelled diffraction peaks are
3
Elemental compositions of the catalysts before and after their
use in the reaction were analyzed by XRF method. The analysis was
performed with powder method by using Spectro IQ II instrument
and CuK␣ radiation.
related to WO3 crystals. The intensities of these peaks increased
with the loading amount and calcination at the lower tempera-
ture. This indicated that the higher calcination temperature favored
well dispersion of WO on Zr-SBA-15 support by transforming the
3
WO3 crystals into smaller particles. Similar observations are also
reported in literature [31].
2.3. Catalytic activity
Additionally, Raman spectroscopy was applied as a sensitive
Cetyl alcohol (CA) and palmitic acid (PA) esterification reactions
technique for detection of very small WO crystallites. The Raman
3
were performed under N atmosphere in a four necked round bot-
spectra of the catalysts is given in Fig. 2. WO /Zr-SBA-15 catalysts
2
3
−
1
tom flask (250 ml) equipped with a Teflon coated magnetic stirring
bar with a stirring rate of 520 rpm and a Dean Stark apparatus sur-
mounted with a reflux condenser. In a typical experiment, 160 mg
of catalyst was added into 25 ml of mesitylene and heated up to
showed a broad band at 976 cm
assigned to W O stretching
in hydrated interconnecting polyoxotungstate clusters, and it is
the characteristic sign for monolayer WO dispersion. These WOx
3
species generally have been related with the formation of strong
Brønsted acid sites under reducing environment. Other bands
observed in the Raman spectra of all catalysts at 807, 716, 324
◦
reaction temperature of 162 C. An equimolar solution of palmitic
acid and cetyl alcohol (6 mmol) in 15 ml of mesitylene at room
temperature was added into the reactor. All the reactions were
carried out for a reaction time of 6 h. In a preliminary set of exper-
iments (Table 1) it was found that the reaction was not controlled
by external diffusion at 520 rpm. Samples taken at regular intervals
were analyzed by Agilent 6890 gas chromatography using Ultra 1
−
1
and 275 cm indicate the existence of WO crystals. The bands
3
−
1
at 807 and 716 cm
bands at 324 and 275 cm
represents the stretching mode, while the
−
1
are attributed to the W O bending
mode and W O W deformation mode, respectively. Moreover, as
the WO3 loading amount increases, the areas of all these bands
increase, showing that both the crystalline and highly dispersed
(
25 m × 0.3 mm) capillary column equipped with FID. The injector

V.N. Mutlu, S. Yilmaz / Applied Catalysis A: General 522 (2016) 194–200
197
Fig. 1. XRD patterns of the catalysts prepared (a) WO3/Zr-SBA15-08 and (b) WO3/Zr-SBA15-10.
WO increase simultaneously. When the effect of calcination tem-
Lewis and Brønsted acid sites. However, as the tungsten loading
increased from 15 wt% to 20 wt%, the number of Lewis acid sites of
the catalysts increased, while B/L ratio decreased (Table 2). Calcina-
tion temperature enhanced both Lewis and Brønsted acid sites. This
was attributed to the increase in crystalline and highly dispersed
WO as observed by XRD and Raman analysis. WO crystals led to
Lewis acid while dispersed WOx sites, clusters are associated with
the Brønsted acid sites [35]. Thus, in the FT-IR spectra the area of
the peaks indicating the Brønsted acid sites increased with calcina-
tion temperature, whereas the area of the peaks assigned to Lewis
acid sites increased with tungsten loading.
3
−
1
perature was considered, the band at 976 cm
indicating the
monolayer dispersion of WO3 enhanced with increasing calcina-
tion temperature. Thus, the calcination temperature affected the
character of the acid sites of the catalysts, since these WOx clus-
ters are associated with the Brønsted acid sites. It can be deduced
from the results of Raman and XRD that the surface W will change
from highly dispersed state into a coexistence of dispersed and
crystalline WO3 as the loading amount increases. The interaction
between the support and WO3 changed with calcination temper-
ature and the monolayer dispersing appears as the calcination
temperature increased.
3
3
The acidities of the catalysts obtained by NH -TPD are given
3
◦
All the catalysts exhibited a type-IV adsorption isotherm with
a hysteresis loop, which correspond to a hexagonal pore system
in Fig. 4. All the catalysts had broad acidity peaks between 200 C
and 400 C indicating that the catalysts had moderate acidity. From
◦
(
results not reported) [28,29]. The textural properties of the cat-
the area under the TPD profiles of the catalysts, total acidity of the
catalysts were determined and given in Table 2. It was found that Zr-
SBA15-08, Zr-SBA15-10 and WZSBA08-15-8 were the most acidic
catalysts.
alysts are given in Table 2. The catalysts were mesoporous. Their
surface areas and pore sizes decreased by WO3 loading and calci-
nation temperature. This was attributed to the presence of WO3
clusters in the pores and on the external surface.
3.2. Esterification of cetyl alcohol and palmitic acid
3
.1.2. Acidic properties
The nature of acid sites of the catalysts was determined by FT-
Cetyl palmitate was the only major product obtained over all
the catalysts. The selectivities to cetyl palmitate were higher than
98% which showed that selectivity was independent of the catalyst
properties.
IR spectra of pyridine adsorbed catalyst samples as shown Fig. 3.
The peaks at 1445 cm
Lewis acid sites, while the peaks at 1540 cm are assigned to the
pyridinium ion bonding to Brønsted acid sites and the peaks at
1
compare the concentrations of Lewis and Brønsted acid sites of the
catalysts, the areas under the bands at 1445 cm and 1540 cm
were related to Lewis and Brønsted acid site concentrations respec-
tively [34]. The results indicated that tungsten loading decreased
−
1
indicate the pyridine coordination on
−
1
−
1
495 cm are related with both Lewis and Brønsted acid sites. To
3.2.1. Influence of Zr and W loading and calcination temperature
The results given in Fig. 5 display the conversion of cetyl alcohol
over the catalysts. Cetyl alcohol amount decreased differently with
reaction time depending on the catalyst. Zr-SBA15-08, ZrSBA15-10
and WZSBA15-08-8 provided higher cetyl alcohol conversions and
−
1
−1
Fig. 2. Raman spectra of the catalysts prepared (a) WO3/Zr-SBA15-08 and (b) WO3/Zr-SBA15-10.

1
98
V.N. Mutlu, S. Yilmaz / Applied Catalysis A: General 522 (2016) 194–200
Table 2
Textural properties and total acidities of the catalysts prepared.
2
B/La
Catalysts
BJH dP (Å)
SBET (m /g)
Acidity (mmol NH3/g catalyst)
Zr content (wt %)
W content (wt %)
Zr-SBA15-08
42.4
35.6
37.6
37.3
25.9
34.4
37.6
37.6
25.1
33.6
600.3
323.4
320.5
299.8
230.0
548.6
318.7
318.7
180.4
148.3
1.122
0.718
1.43
0.684
1.000
1.069
0.713
0.996
0.679
0.966
1.39
1.22
1.29
0.71
1.05
1.37
1.21
1.26
0.72
0.90
6.0
11
8.3
12.5
8.6
–
WZSBA08-15-7
WZSBA08-15-8
WZSBA08-20-7
WZSBA08-20-8
Zr-SBA15-10
WZSBA10-15-7
WZSBA10-15-8
WZSBA10-20-7
WZSBA10-20-8
7.7
4.9
10.1
7.0
–
7.5
4.6
9.8
8.5
8.0
13.2
10.6
14.1
12.7
a
−1
−1
B/L defined as the ratio of the areas under the peaks at 1540 cm to 1445 cm
.
Fig. 3. FTIR spectra of pyridine adsorption on (a) Zr-SBA15-08 and (b) Zr-SBA15-10 based catalysts.
Fig. 4. NH3-TPD of the catalysts prepared, (a) Zr-SBA15-08 and (b) Zr-SBA15-10 based catalysts.
Fig. 5. Catalytic results for the esterification of cetyl alcohol and palmitic acid over (a) Zr-SBA15-08 and (b) Zr-SBA15-10 based catalysts.

V.N. Mutlu, S. Yilmaz / Applied Catalysis A: General 522 (2016) 194–200
199
Table 3
◦
Initial rate of disappearance and conversion of cetyl alcohol and yield of cetyl palmitate over different catalysts at 162 C after 6 h reaction time.
Catalysts
r0 × 105 (mol/min g)
r0/TAa(mol/min mmol NH3)
Conversion (%)
Yield (%)
Zr-SBA15-08
9.23
3.63
6.88
2.75
6.40
8.33
3.88
4.31
3.46
4.17
8.23
5.05
4.81
4.02
6.40
7.43
5.44
4.33
5.10
4.31
64.3
24.5
54.6
22.3
38.2
62.7
23.9
33.5
22.5
34.6
63.1
24.1
53.8
21.9
37.9
62.0
23.6
33.0
22.1
34.3
WZSBA08-15-7
WZSBA08-15-8
WZSBA08-20-7
WZSBA08-20-8
Zr-SBA15-10
WZSBA10-15-7
WZSBA10-15-8
WZSBA10-20-7
WZSBA10-20-8
a
r0/TA is defined as r0 × 105 per total acidity.
Table 4
Table 5
Influence of temperature and feed composition on the initial rate of disappear-
ance and conversion of cetyl alcohol and yield of cetyl palmitate over Zr-SBA15-08
catalyst after 6 h reaction time.
Influence of catalyst amount on the initial rate of disappearance and conversion of
cetyl alcohol and yield of cetyl palmitate over Zr-SBA15-08 catalyst after 6 h reaction
time.
Temperature ( C) CA/PA r0 × 105 (mol/min g) Conversion (%)
◦
Yield (%)
41.1
63.1
83.3
Catalyst (%)a
r0 × 105 (mol/min)
Conversion (%)
Yield (%)
135
162
162
1/1
1/1
1/2
4.94
9.23
13.00
42.6
64.3
83.8
2.67
5.35
10.69
0.46
1.48
3.75
41.2
64.3
98.9
41.0
63.1
98.3
a
Percentage of catalyst related to the total weight of reactants.
cetyl palmitate yields as shown in Table 3. These catalysts had the
highest total acidity and the highest amount of Brønsted acid sites.
ZrSBA15-08 and Zr-SBA15-10 gave maximum conversions of 64.3
and 62.0%, respectively.
Table 6
Zr and W contents of fresh and used catalysts.
Catalysts
Zr content (wt%)
W content (wt%)
The catalyst activities were assessed by calculating initial disap-
pearance rate of cetyl alcohol (ro, mol/gcat min) and initial rate per
Fresh catalyst Used catalyst Fresh catalyst Used catalyst
Zr-SBA15-08
7.91
7.52
7.97
7.92
–
7.74
–
–
5.98
–
acid site (ro/TA, mol/mmol NH min). They are tabulated in Table 3.
3
WZSBA08-15-8 8.30
Zr-SBA15-10 8.03
The catalyst with high acidity gave high initial rate. Increasing Zr
loading decreased initial rate. This was the result of the drop in the
acidity of the catalyst observed. Loading tungsten further decreased
initial rate of Zr-SBA-15 catalysts. Even more tungsten loading also
reduced the initial rate. This was attributed to decrease in the acid
hol and palmitic acid. Since esterification is a reversible reaction,
using an excess amount of one reactant shifts the reaction towards
the products. The results of the catalytic tests using CA/PA ratio
of 1/1 and 1/2 over Zr-SBA15-08 catalyst are given in Table 4. As
expected, using an excess amount of palmitic acid increased both
the initial disappearance rate of cetyl alcohol and the yield of cetyl
palmitate substantially. Therefore, the process should be optimized
by taking into account the yield of cetyl palmitate and the cost of
excess reactants. Separation and recycling of excess reactant might
be considered as a solution to achieve the optimum conditions.
sites as a result of WO crystals formation which was discussed in
3
the catalyst characterization section. However, rising calcination
◦
temperature from 700 to 800 C, improved initial rate of reaction
and cetyl alcohol yield. This was in line with the catalyst charac-
terization results; as the higher calcination temperature enhanced
acidity by decreasing WO crystal size and providing the monolayer
3
dispersion of WOx species which resulted in the improvement of
the Brønsted acid sites.
Even though WZSBA08-15-8 catalyst had higher acidity than Zr-
SBA15-08 and Zr-SBA15-10, it gave a lower initial rate per active
site, −ro/TA. This could be due to its lower Brønsted acid sites con-
tent, see B/L ratio given in Table 2. Therefore, it can be concluded
that Brønsted acid sites were more effective for the reaction. The
overall activities observed were found to be affected by catalyst
deactivation behavior as well.
3
.2.4. Influence of catalyst amount
Catalyst amount was studied as another reaction parameter for
the esterification of cetyl alcohol and palmitic acid. The catalytic
◦
tests were performed over Zr-SBA15-08 catalyst at 162 C and the
results are given in Table 5. The amount of catalyst raised the initial
disappearance rate of cetyl alcohol and the yield of cetyl palmitate
significantly. For example, conversion increased from 64.3 to 98.9%
when a catalyst amount changed from 5.35% to 10.69%. An ester
yield of 98% was obtained for 10.69% catalyst amount.
3
.2.2. Influence of temperature
The reaction was also performed at 135 C over Zr-SBA15-08
◦
catalyst in order to investigate the influence of temperature. The
initial activities, cetyl palmitate yield and conversion obtained are
◦
◦
compared with these at 162 C in Table 4. An increase of 27 in
the reaction temperature produces almost two-fold increase in the
initial disappearance rate of cetyl alcohol. Moreover, the conversion
of cetyl alcohol and the yield of cetyl palmitate also increased by
about 21%. These significant changes with temperature indicates
that the reaction is kinetically controlled under conditions studied.
3.2.5. Catalyst stability and reusability
The reusability of the three most active catalysts (Zr-SBA15-08,
Zr-SBA15-10 and WZSBA08-15-8) was tested up to three cycles. The
results are given in Fig. 6. The activity of WZSBA08-15-8 catalysts
decreased quite substantially in the second reuse. This could be
due to the significant leaching of WO3 species which was found
from the results of XRF analysis given in Table 6. Although there
was some Zr leaching, Zr-SBA15-08 catalyst showed no significant
decrease in its activity up to 3rd reuse.
3.2.3. Influence of feed composition
From the economic point of the process, it is important to obtain
high cetyl palmitate yield by using equimolar amounts of cetyl alco-

2
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V.N. Mutlu, S. Yilmaz / Applied Catalysis A: General 522 (2016) 194–200
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