biodiesel

1. Use of HZSM-5 modified with citric acid as acid heterogeneous catalyst
for biodiesel production via esterification of oleic acid
Sara S. Vieira a
, Zuy M. Magriotis a,⇑
, Maria Filipa Ribeiro b
, Inês Graça b
, Auguste Fernandes b
,
José Manuel F.M. Lopes b
, Sabrina M. Coelho a
, Nadiene Ap.V. Santos a
, Adelir Ap. Saczk a
a
Departamento de Química, Universidade Federal de Lavras, 37.200-000 Lavras, MG, Brazil
b
IBB, Institute for Biotechnology and Bioengineering, Centre for Biological and Chemical Engineering, Instituto Superior Técnico – Universidade de Lisboa, Av. Rovisco Pais,
1049-001 Lisboa, Portugal
a r t i c l e i n f o
Article history:
Received 16 June 2014
Received in revised form 1 September 2014
Accepted 5 September 2014
Available online 16 September 2014
Keywords:
Biodiesel
Esterification
HZSM-5
Dealumination
Citric acid
a b s t r a c t
In this study, the efficiency of citric acid as dealumination agent was evaluated in the case of zeolite
HZSM-5 by varying the experimental conditions, namely concentration (0.5, 1.0 and 2.0 mol LÀ1
) and
treatment temperature (60 and 80 °C). The effect of the CA treatment on structural, textural and acidity
properties of the starting HZSM-5 material was monitored using XRD, N2 sorption and FTIR (pyridine and
collidine) measurements. Heptane and methylcyclohexane cracking model reactions were used to eval-
uate the catalytic behavior of the modified materials, in terms of acid strength and active sites accessi-
bility. The modified catalysts were tested in the esterification reaction, using oleic acid as reagent and
different oleic acid/methanol molar ratios. All the results show the beneficial effect of the citric acid treat-
ment on the physicochemical properties of the final materials, with an improvement of the external acid-
ity, a crucial parameter for the reaction considered, i.e. the oleic acid esterification.
Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction
Increasing concerns related to, mainly, environmental issues
have led to enhanced interest in the search for renewable sources
in the field of transportation fuels production [1,2]. A remarkable
intensified research has been directed, in fact, to biomass derived
fuels, aiming to gradually reduce the dependence from crude oil
and carbon dioxide emissions [3,4]. Additionally, relevant benefits
should result from the absence of sulfur compounds in the biofuel
[5]. In practice, biodiesel increased, in recent years, its market
share through blending with petrodiesel.
Biodiesel is a mixture of long chain fatty acid alkyl esters,
obtained from the transesterification of triglycerides and esterifi-
cation of free fatty acids (FFA) with alcohols [3,6,7], contained in
vegetable oils and other types of fat feedstocks. It is currently pro-
duced using homogeneous mineral, acid or basic (NaOH) catalysts,
presenting these processes interesting reaction rates for triglycer-
ides transesterification and some undesired features: corrosion
problems; expensive process steps of catalyst/products separation;
need to remove FFA and water from the feedstock, in order to pre-
vent catalyst consumption and aqueous quenching [8].
Many efforts have been then carried out in order to develop
more eco-friendly heterogeneous processes effective for biodiesel
production [2,9], either basic or acid solids being potentially inter-
esting recyclable catalysts. Several studies report the attempting
use of rare-earth and other types of oxides [10–16], clays [17]
and zeolites [16,18,19]. Some minimization of the homogeneous
process drawbacks can effectively result by using heterogeneous
base-catalyzed systems: easy separation processes, possibility of
use continuous flow reactors, catalyst recycling, etc. Solid acid cat-
alysts have shown to be more interesting [20], as they combine the
advantages of the solid materials and the mineral acids [21]. They
revealed to be able to simultaneously promote the esterification
and transesterification reactions [21], which is especially interest-
ing for the transformation of oils containing high amounts of FFA,
the feedstock pretreatment becoming unnecessary, thus involving
reduced costs. However, some limitations of the solid acid cata-
lyzed biodiesel production still remain, such as the promotion of
secondary reactions and the observed kinetics.
The specific research concerning the production of biodiesel
from long chain FFA esterification has been also intensified [22].
The oleic acid, present in several vegetable oils, despite being one
of the most common FFA, has been used in few esterification
http://dx.doi.org/10.1016/j.micromeso.2014.09.015
1387-1811/Ó 2014 Elsevier Inc. All rights reserved.
⇑ Corresponding author. Tel.: +55 35 38291889; fax: +55 35 38291812.
E-mail addresses: saraufla@yahoo.com.br (S.S. Vieira), zuy@dqi.ufla.br
(Z.M. Magriotis), filipa.ribeiro@ist.utl.pt (M.F. Ribeiro), ines.sofia.graca@ist.utl.pt
(I. Graça), auguste.fernandes@ist.utl.pt (A. Fernandes), jmlopes@ist.utl.pt
(J.M.F.M. Lopes), sabrina10mcoelho@yahoo.com.br (S.M. Coelho), nadi.ene@
hotmail.com (N.Ap.V. Santos), adelir@dqi.ufla.br (A.Ap. Saczk).
Microporous and Mesoporous Materials 201 (2015) 160–168
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2. studies [19]. Similarly to other cases [2,21], the conversion of oleic
acid revealed to be efficiently promoted by solid acid catalysts
[19,23]. Several features of these materials should be determinant,
such as high strength and concentration of acid sites and high sur-
face area with hydrophobic character [2,21]. These and other prop-
erties constitute important and well-known characteristics of
zeolites, which are microporous crystalline aluminosilicates widely
used in heterogeneous catalysis [24]. Additionally, the acid/base
properties, the hydrophilic/hydrophobic character, the porous net-
work properties, of zeolites can be finely tuned, so that specific lev-
els of Brønsted/Lewis acid sites strength and concentration,
hydrophobic selective adsorption ability, etc., can be achieved, thus
allowing possible activity and selectivity enhancements towards
esterification reactions [25].
Zeolites ZSM-5 and Mordenite have shown to be interestingly
active in the esterification of oleic acid with methanol [19], with
special emphasis for ZSM-5 [23]. This catalyst presents an internal
pore network with apertures of approximately 5.5 Å, thus prevent-
ing the inner penetration of oleic acid, and also very strong
Brønsted acid sites located in a generally hydrophobic surface.
Several post-synthesis treatments have been widely applied
over zeolites, and specifically over ZSM-5, namely the controlled
dealumination [25–27], searching for more efficient catalysts.
The typical Brønsted acidity present in zeolites arises from the
hydroxyl (Si–(OH)–Al) groups bridging two adjacent tetrahedra,
SiO4 and AlO4
À
, of its aluminosilicate framework. The concentration
of these sites, as well as, in many cases, their acid strength, would
be directly related to the aluminum content of the structure. The
removal of framework aluminum by dealumination treatments,
which may be performed by steaming or acid leaching, for exam-
ple, will then change the amount (and eventually the strength) of
Brønsted acid sites, and possibly create some Lewis sites. Simulta-
neously, some mesoporosity is usually generated, promoting in
this way an enhancement of the remaining acid sites accessibility,
which will be of particular relevance when testing large molecules
transformation over ZSM-5 zeolite. The hydrophobic character of
the zeolite surface will be also modified, by controlling the nega-
tive charge in the structure associated with framework Al species
[28].
Inorganic strong acids, such as HCl or HNO3, are generally used
in zeolites Al leaching. However, depending on the severity of the
treatment and the characteristics of the zeolite, serious damage of
the zeolite structure may occur. The use of citric acid as dealumi-
nation agent seems to be of special interest in order to produce
the mentioned modifications in the more relevant external layer
of the catalyst particles, since the transformation of the oleic acid
promoted by ZSM-5 will proceed over the external surface of the
zeolite crystal. In fact, due to the large dimensions of this acid mol-
ecule, it was reported that even over a larger pore USY zeolite, the
dealumination took place gradually from the outer to inner sites of
the crystallite. The preferential mesopore generation at the exter-
nal surface of the solid particle has been pointed out as a positive
factor [29].
The main goal of the present study is to produce interesting
modifications on a HZSM-5 zeolite by citric acid (CA) treatment,
so that its catalytic properties towards the oleic acid esterification
with methanol, a model reaction that is used to simulate the bio-
diesel production, could be improved. A detailed characterization
of the parent and treated zeolite samples was pursued, in order
to monitor the induced modifications, namely, in the nature, num-
ber and strength of the acid sites, as well as in their accessibility,
trying then to correlate it with the catalytic changes. The Brønsted
acidity was characterized by infrared spectroscopy (FTIR) of the OH
stretching groups; the total (and external) acidity was character-
ized by pyridine (and collidine) adsorption followed by FTIR; the
textural properties were evaluated from nitrogen sorption studies.
Additionally, the HZSM-5 samples have been tested in the model
reactions of heptane and methylcyclohexane cracking, searching
for monitoring, in reaction conditions, also the acidity of the sam-
ples and the sites accessibility. In fact, heptane transformation
demands higher strength of the acid sites than methylcyclohexane,
since the corresponding intermediary carbenium ions involved in
the cracking mechanism are less stable in the former case [30].
On the other side, heptane can penetrate much more easily inside
the HZSM-5 porous network than methylcyclohexane, which is a
bulky molecule.
2. Experimental
2.1. Preparation of catalysts
Zeolite HZSM-5 (MFI structure) used in this work as parent
sample was supplied by Zeochem (reference n° V1148.4) with a
Si/Al ratio of 14. It was then modified using citric acid (CA). The
effect of both treatment temperature (60 and 80 °C) and CA con-
centration (0.5; 1.0 e 2.0 mol LÀ1
) was evaluated. Typically, a sus-
pension of zeolite sample into an aqueous solution containing CA
(with the appropriate concentration) was prepared and the mix-
ture was then heated under stirring for 3 h [31]. Afterwards, the
final product was recovered by filtration, washed several times
with distillated water and finally dried in an oven at 100 °C for
24 h. The resulting solid was calcined at 500 °C in a quartz reactor
for 4 h (heating rate of 10 °C minÀ1
), under synthetic air flow
(100 mL minÀ1
). Hereafter, the samples will be referred to as HZ/
X/Y where X and Y stand for, respectively, CA concentration and
treatment temperature.
2.2. Characterization of catalysts
The chemical composition of the different materials, i.e. the Si/
Al molar ratio, was determined by XRF analysis performed on a
Spectrometer X-ray Philips PW1480 (Rh Tube Anode), using a cal-
ibration curve obtained from different zeolite references (different
Si/Al ratios).
The structural characterization of the parent and modified HZ
samples was carried out by Powder X-ray diffraction using a D8
Advance diffractometer from Bruker, equipped with a graphite
monochromator using Cu-Ka radiation (1.5406 Å) as incident
beam and operating at 40 kV and 40 mA. The diffraction patterns
were obtained from 5° to 65° (2h), with a step size of 0.05 and a
time per step of 3 s.
Textural characterization of the solids was carried out by means
of N2 sorption measurements isotherms at À196 °C, performed in
an automatic apparatus Micromeritics ASAP 2010, where the sam-
ples were outgassed at 350 °C during 4 h under vacuum before N2
sorption measurements. The micropores volume (Vmicro) and exter-
nal surface (Sext) area were determined using t-plot method. Total
pores volume (Vpore) was determined at pressure P/P0 of 0.97
(adsorption branch).
The samples acidity was characterized by pyridine and collidine
(basic probe molecules) adsorption followed by IR spectroscopy,
using a Nicolet Nexus spectrometer. The samples were pressed into
thin wafers (10–20 mg cmÀ2
) and pre-treated in an IR quartz cell at
450 °C for 3 h under secondary vacuum (10À6
mbar). The samples
were then cooled down to 150 °C and contacted with the probe
molecule. After that, the excess of probe molecule was evacuated
for 30 min under secondary vacuum and then IR spectra were
recorded (64 scans with a resolution of 4 cmÀ1
). The background
spectrum, always recorded under identical conditions without
sample and performed before each spectrum acquisition, was auto-
matically subtracted. In what concerns pyridine experiment, the
S.S. Vieira et al. / Microporous and Mesoporous Materials 201 (2015) 160–168 161

3. samples were subsequently heated at 250, 350 and 450 °C under
vacuum (30 min for each temperature) and the respective spectra
acquired.
For quantification purposes, the spectrum of the sample after
pretreatment was subtracted from the spectra obtained after the
probe molecule adsorption and subsequent desorption. Brønsted
and Lewis acid sites concentrations were determined from
pyridine adsorption, using areas of bands at 1545 and 1450 cmÀ1
,
respectively. Molar absorptivity used were 1.13 and
1.28 cm lmolÀ1
for pyridine interacting with Brønsted and Lewis
centers, respectively [32]. Total external acidity was evaluated
using collidine as probe molecule. Quantification was carried out
using the integrated area in the region 1600–1700 cmÀ1
; molar
absorptivity used was 10.1 cm lmolÀ1
[33].
2.3. Heptane and methylcyclohexane catalytic tests
HZSM-5 samples obtained through citric acid dealumination
were also characterized in what concerns their acidity and accessi-
bility by performing two model hydrocarbon transformations, the
heptane transformation, a linear alkane whose transformation is
extremely dependent on the acidity strength, and the methylcyclo-
hexane transformation, a bulky molecule for which diffusion into
the HZSM-5 porous network is limited [34,35].
Before the reaction, all catalysts were pre-treated at 450 °C
under nitrogen flow (60 mL minÀ1
) for 8 h. The catalytic tests were
carried out in a Pyrex fixed-bed reactor, at 450 °C, under atmo-
spheric pressure. The reactor feed was composed by 10 mol% of
reactant (heptane or methylcyclohexane, Merck, 99%) and
90 mol% of N2. The reactant flow rate was maintained constant
(4 mL hÀ1
) with a B. Braun compact perfusor and the nitrogen flow
(120 mL minÀ1
) was controlled by a Brooks Instrument controller.
The reactor effluent samples were taken for different time-
on-stream (TOS): 2, 5, 10, 15, 30 and 60 min, using a 10-position
valve supplied by Vici. The first TOS value was the time necessary
to get a stable reactant pressure in the reactor. The tests were
performed for a contact time (s = 1/WHSV) of 4 min, using about
200 mg of catalyst. The reaction products were analyzed by a
Shimadzu GC-14B gas chromatograph with a Plot Al2O3/KCl fused
silica capillary column (50 m), using a flame ionization detector
(FID) and nitrogen as carrier gas. The heptane and methylcyclohex-
ane conversions were estimated using the areas of the chromato-
graphic peaks (A):
Conversion ð%Þ ¼ 1 À
AReactant
ATotal
 100
2.4. Oleic acid esterification catalytic tests
In order to simulate the biodiesel production by esterification,
the oleic acid conversion into methyl oleate (methylic route) in
presence of methanol was used as model reaction. The experi-
ments were performed under autogenous pressure, in a 20 mL
glass batch reactor at 100 °C. Increased quantities of oleic acid
(Vetec, 99%) were added to 5 mL of methanol (J.T. Baker, 99.9%),
in order to change the oleic acid/alcohol ratio: 1:10, 1:20, and
1:45. The system was maintained under continuous stirring and
the reaction temperature was kept constant using a silicon bath.
The conditions used to obtain the highest oleic acid conversion
were: 10% of catalyst relatively to the quantity of oleic acid used
and 4 h. To determine the conversion, 10 lL of the reaction mixture
were collected and diluted into 1 mL of hexane (Synth, 98.5%), and
50 lL of an internal standard (Tricaprylin – Sigma Aldrich, 99%)
were added. Then, the solution was analyzed in an Agilent
Technology 7890A gas chromatograph, equipped with a FID
detector. The capillary column used was an HP-5 Agilent
(30 m  0.320 mm  0.25 lm), with a polar phase composed by
5% phenyl methyl siloxane. The quantification was performed by
internal standard. The conversions of oleic acid were determined
according to:
Conversion ð%Þ ¼ 1 À
NFinal-ester
NTotal-ester
 100
where NTotal-ester is the total number of moles of methyl oleate that
should be formed according to the reaction stoichiometry and
NFinal-ester the total number of moles of methyl oleate quantified
by CG/FID after 4 h. It is important to refer that the methyl oleate
was the only product detected in the reactor effluent.
3. Results and discussion
3.1. Catalysts characterization
The Si/Al ratio determined from XRF analysis are listed in
Table 1. Logically, parent HZ zeolite shows the lower Si/Al value
(11.4), i.e. the higher amount of aluminum, while all the acid-
treated samples show slightly higher Si/Al ratios (13.9–15.0). For
commodity, XRF results will be discussed more in details together
with the others physicochemical and catalytic results.
Fig. 1 presents the X-ray powder diffractogram obtained for
sample HZ. The latter presents a set of diffraction peaks that corre-
sponds to MFI structure [36]. The peaks are well defined and very
intense, suggesting a high crystalline sample.
Table 1 presents the percentage of crystallinity calculated for
the different samples, using the area of the peaks between 22°
and 25° (2h) and comparing with the area corresponding to the
sample HZ, considered as a reference. The range considered for
the peaks integration, in the case of zeolites with MFI structure,
was used to minimize the influence of sample hydration [37]. It
can be seen that treatment with citric acid does not influence dras-
tically the crystallinity of the different samples, as values between
90% and 100% are obtained.
Fig. 2 presents the sorption isotherm of N2 performed at
À196 °C for the parent HZSM-5 and some samples modified. As
the other modified HZSM-5 samples present similar profiles, they
have not been shown.
The isotherm in Fig. 2 shows the simultaneous presence of
micropores (high initial N2 uptake at P/P0 0.1) and mesopores
that can be evidenced by the presence of a hysteresis loop at higher
P/P0 (close to 1) [38]. The total uptake at high P/P0 can be controlled
by three mechanisms (1) intracrystalline filling at low P/P0; the
multilayer adsorption on the external surface area; and (3) capil-
lary condensation within a secondary pore structure [39]. This type
of isotherm has been assigned by the presence of interparticle mes-
opores created by agglomeration of small crystallites. The surface
of these mesopores (external surface of the crystallites) can contain
active centers [40]. The experimental data were treated according
to [41] (t-plot method) and allowed to provide external area (Sext),
micropores volume (Vmicro) and total pore volume (Vpore) of the dif-
ferent samples.
The results are presented in Table 1. First, it can be seen that HZ
dealumination with citric acid has almost no influence on the final
micropores volumes. However, samples modified with citric acid
show slightly higher total pore volumes (except HZ/0.5/80) and
external surface areas. For HZSM-5, the mesopores contribution
was estimated about 25% to the total pore volume [42]. A similar
trend was verified in a previous study [31] where the authors con-
clude that optimal dealumination conditions were acid citric con-
centration of 1.0 mol LÀ1
and 80 °C. Indeed, textural properties
modifications need to be balanced with the amount of final acid
162 S.S. Vieira et al. / Microporous and Mesoporous Materials 201 (2015) 160–168

4. sites as a too severe dealumination could destroy partially the
structure, leaving only a few strong acid sites necessary for the
reaction studied.
Also, Xin-Mei et al. [29] showed that during acid treatment of Y
zeolite, citric acid molecules formed complexes with Al, giving rise
to the widening of the micropores opening essentially at the exter-
nal surface (because of the size of the citric acid) and that an opti-
mal acid concentration was also needed to optimize the final
zeolite textural and acidic properties.
Regarding the temperature at which the acid treatment was
carried out, it may be inferred that this parameter has no influence
in what concerns HZ dealumination, except for the treatment at
1.0 mol LÀ1
. As referred above, sample HZ/1.0/60 showed the high-
est improvement in terms of external surface area (77 m2
gÀ1
).
Fig. 3 presents the infrared spectra obtained for the different
samples, in the region of the OH groups (3900–3400 cmÀ1
). Differ-
ent OH species can be observed: bridging hydroxyls (Si–(OH)–Al)
(3608 cmÀ1
), terminal silanols Si–OH (3743 cmÀ1
) [43], and finally
OH groups originated from Al species, extra-framework aluminum
species (EFAL) and tri-coordinated Al species connected to struc-
ture oxygen atoms (respectively 3662 and 3780 cmÀ1
) [44,45],
thus indicating the existence of an important amount of EFAL spe-
cies in these samples. Normally, these EFAL species are related to
Lewis acid centers and do not interfere with the other acid sites,
i.e. Brønsted sites, although Al Lewis species have been shown to
enhance the acid strength of the protonic centers [46].
From what it can be observed in Fig. 3, CA treatment using dif-
ferent concentrations and temperatures originates small altera-
tions but detectable. Concerning the band at 3608 cmÀ1
, one can
observe a slight decrease of the band with the acid treatment, i.e.
a decrease of (Si(OH)–Al) groups; in the case of HZ/1.0/80 sample,
this decrease is about 23%, which means that temperature has,
here, an important effect. For the other samples, the decrease of
the band at 3608 cmÀ1
is less pronounced: 9%, 3% and 1% for HZ/
0.5/60, HZ/1.0/60 and HZ/2.0/60, respectively, indicating that
temperature of 60 °C has no or small effect on the intensity of
the (Si–(OH)–Al) band.
Also, it may be noted that band at 3743 cmÀ1
, relative to iso-
lated silanols (Si–OH) groups increases logically with CA treat-
ment, as more surface is created [47] during dealumination
(except for HZ/1.0/80). OH bands related to extra-framework Al
species (3662 and 3780 cmÀ1
) also suffered slight reduction during
dealumination as citric acid can remove EFAL species by complex-
ation and thus reopens channels and pores of the HZ structure [48].
In Fig. 3, it can be also seen how the OH region of the modified
HZ catalysts is altered after pyridine adsorption. For example, the
band at 3608 cmÀ1
disappears completely, confirming the acidic
character of the bridging Si–(OH)–Al species. On the other hand,
the band from EFAL species (3668 cmÀ1
) decreases, but not
completely. Indeed, the band remaining at about 3680 cmÀ1
after
pyridine adsorption might correspond to specific EFAL species pre-
senting a different acidic character and/or accessibility. Logically,
the band from isolated silanols (weak acid sites) at 3743 cmÀ1
seems to be unaffected, or a little, by the presence of adsorbed
pyridine. Finally, the band at 3780 cmÀ1
, corresponding to OH from
tri-coordinated Al species, (also Lewis acid centers) vanishes
completely.
Table 1
Si/Al ratio, percentage of crystallinity and textural parameters calculated for the different samples.
Catalyst Si/Ala
Crystallinity (%) Vmicro (cm3
gÀ1
)b
Vpore (cm3
gÀ1
)b
Sext (m2
gÀ1
)b
HZ 11.4 100 0.14 0.45 69
HZ/0.5/60 14.4 97 0.15 0.49 71
HZ/0.5/80 14.6 97 0.14 0.46 73
HZ/1.0/60 13.9 95 0.15 0.50 77
HZ/1.0/80 14.6 96 0.15 0.49 73
HZ/2.0/60 14.0 94 0.14 0.49 74
HZ/2.0/80 15.0 95 0.15 0.48 72
a
From XRF analysis.
b
From t-plot.
10 20 30 40 50 60
(HZ/1.0/80)
(HZ/1.0/60)
(HZ/0.5/60)
2θ (degree)
Countspersecond
(HZ)
Fig. 1. Powder X-ray pattern of parent HZSM-5 and treated samples.
HZ
50 cm
3
g
-1
Adsorbedvolume(cm3
g-1
)STP
HZ/0.5/60
HZ/1.0/60
HZ/1.0/80
P/P0
0.0 0.2 0.4 0.6 0.8 1.0
Fig. 2. N2 adsorption–desorption isotherm obtained for parent HZ sample: being -
h- adsorption isotherm and -s- desorption isotherm.
S.S. Vieira et al. / Microporous and Mesoporous Materials 201 (2015) 160–168 163

5. Fig. 4 shows the spectra of parent HZSM-5 sample after pyridine
adsorption and subsequent desorption at different temperatures,
in the zone of the pyridine ring frequencies (1700–1350 cmÀ1
).
The spectra of the other samples modified by CA treatment also
show the same set of bands, although with different intensities.
In the spectrum a) (Fig. 4), bands from pyridinium ions (interaction
with Brønsted acid sites) at 1630 and 1540 cmÀ1
and from pyridine
adsorbed onto Al Lewis sites at 1620–1600 and 1455 cmÀ1
can
be observed. The band observed at about 1490 cmÀ1
is common
to the two pyridine species [49,50]. Using the integrated areas of
the bands at 1545 and 1455 cmÀ1
, respectively, allowed the quan-
titative determination of the number of Brønsted and Lewis acid
sites presents in each sample. The molar absorptivity (e) used were
taken from literature [32]. The pyridine desorption at different
temperatures allowed to evaluate the strength of the respective
acid sites. Collidine (2,4,6-trimethylpyridine) was also used in
order to determine quantitatively the amount of Brønsted acid
sites presents at the external surface of the materials [33]; in fact
the larger size of collidine when compared with pyridine (hin-
drance effect), does not allow the molecule to either interact with
Lewis acid sites or with Brønsted acid sites present at the interior
of the MFI structure pores [51].
Table 2 summarizes the different quantitative parameters cal-
culated, namely Brønsted and Lewis acid sites concentration ([B]
and [L] from pyridine), overall Brønsted to Lewis ratio [B]/[L], ratios
B]450/[B]150 and [L]450/[L]150 to evaluate acid sites strength and
finally external acidity concentration (from collidine).
3900 3800 3700 3600 3500 3400
Wavenumber (cm
-1
)
Absorbance(a.u.)
3780 3743 3662
HZ
HZ/0.5/60
HZ_2.0_60
HZ/1.0/80
HZ/1.0/60
3608
3850 3800 3750 3700 3650 3600 3550
Wavenumber (cm
-1
)
Absorbance(a.u.)
a
b
HZ
3850 3800 3750 3700 3650 3600 3550
Absorbance(a.u.)
Wavenumber (cm
-1
)
HZ/0.5/60
b
a
3850 3800 3750 3700 3650 3600 3550
Wavenumber (cm
-1
)
Absorbance(a.u.)
HZ/1.0/60
b
a
3850 3800 3750 3700 3650 3600 3550
Absorbance(a.u.)
Wavenumber (cm
-1
)
HZ/1.0/80
b
a
3850 3800 3750 3700 3650 3600 3550
Wavenumber (cm
-1
)
Absorbance(a.u.)
b
a
HZ/2.0/60
Fig. 3. FTIR spectra in the region of the OH groups and Py-FTIR spectra in the region of the hydroxyl groups of HZ and modified CA samples (a) before and (b) after adsorption
of pyridine at 150 °C.
1700 1650 1600 1550 1500 1450 1400 1350
Absorbance(a.u.)
Wavenumber (cm
-1
)
d
c
b
a
HZ
Fig. 4. FTIR spectra in the region 1700–1350 cmÀ1
after saturation with pyridine
and subsequent desorption at 150 °C (a), 250 °C (b), 350 °C (c) and 450 °C (d) for the
parent HZ.
164 S.S. Vieira et al. / Microporous and Mesoporous Materials 201 (2015) 160–168

6. From Table 2, it can be observed that dealumination caused a
reduction of both Brønsted and Lewis acid sites which is in line
with XRF analysis that showed an increase of the Si/Al ratio, i.e. a
decrease of Al content, for all the materials modified with CA treat-
ment. Comparing the samples obtained for the CA treatment per-
formed at 60 °C, one can see that sample HZ/0.5/60 shows the
most important reduction (21% less acid sites when compared with
the parent zeolite), while for sample HZ/1.0/60, the reduction is
only 7%. This is well in line with XRF results as HZ/0.5/60 shows
the higher Si/Al value of 14.4. It can be concluded that, although
the number of acid sites always decreases with the CA treatment,
this reduction is not linear with the CA concentration used. Con-
cerning the effect of the treatment temperature, we can see that
higher temperature (80 against 60 °C) leads to a higher reduction
of the number of the acid sites (about 30% when comparing with
the parent zeolite). Indeed HZ/1.0/80 shows the highest Si/Al value,
i.e. 14.6.
In what concerns Lewis acidity, we can see that samples
HZ/1.0/60 and HZ/1.0/80 are those where a significant decrease
of Lewis acid sites is observed, that is, there exist optimal condi-
tions (concentration and temperature) for the removal of EFAL Al
species.
An interesting fact is about the external acidity calculated from
collidine adsorption measurements. In Table 2, it can be seen that
external acidity is higher for samples modified with CA treatment,
compared with parent HZ sample, being the maximum attained for
the CA treatment performed at 60 °C (about 29% increase). This
increase of external acidity for the samples modified may be
explained by the increase of external surface area during CA
treatment that allow new acid sites to be accessible for collidine
molecule. This assumption is confirmed in Fig. 5 where a linear
relationship is observed between the external surface area
obtained from N2 adsorption (t-plot method) and the external
acidity determined by collidine adsorption for the different
samples.
3.2. Heptane and methylcyclohexane transformations
Fig. 6a shows the initial heptane conversions (after 2 min TOS)
found for the parent and dealuminated samples. It can be seen that,
despite the slight increase of the total pore volume observed for all
the dealuminated samples (Table 1) and, so, the improved mole-
cules diffusion inside the zeolite porous network, the dealumina-
tion treatment induced a decrease of the conversion for the HZ/
0.5/60 and HZ/1.0/80 zeolites. This can be explained by the much
more important reduction of the concentration of the Brønsted
acid sites observed for these samples, when compared with the
parent zeolite and with the other dealuminated samples (Fig. 6b).
Therefore, for these two samples, due to the acid demanding char-
acter of the heptane transformation, the impact on the conversion
of the decrease of the Brønsted acid sites amount prevails upon the
beneficial effect of enhancing the total pore volume and the acidity
strength of the active sites (Table 2). On the other hand, for the HZ/
1.0/60 and HZ/2.0/60 samples, an increase of the conversion was
noticed, relatively to the parent zeolite. In this case, as the decrease
the number of Brønsted acid sites caused by the dealumination
was not as significant as for the other two dealuminated zeolites
and as the HZ/1.0/60 and HZ/2.0/60 zeolites are the samples pre-
senting the highest Brønsted acidity strength (Table 2) and the
greatest improvement of the total pore volume (Table 1), better
conversion values were obtained.
On the other hand, concerning the results obtained with meth-
ylcyclohexane (Fig. 7), it can be seen that, in spite of the decrease of
the total number of Brønsted acid sites with the dealumination
(Table 2), there is an increase of the activity for all dealuminated
samples (Fig. 7a), which can be mainly correlated with the
enhancement of the external acidity due to the acid treatment, as
it can be seen in Fig. 7b. This is not strange considering that the dif-
fusion of bulky molecules, such as methylcyclohexane, through the
HZSM-5 zeolites channels is, usually, limited by the space avail-
able, i.e. the dimensions of the channels. In fact, methylcyclohex-
ane has a kinetic diameter (5.8 Å) that is slightly higher than the
size of the HZSM-5 pore openings (5.4 Â 5.6 Å–5.1 Â 5.4 Å). This
can also explain the much lower conversion values found for meth-
ylcyclohexane than for heptane, considering that, in absence of
diffusional constraints, methylcyclohexane, being a more reactive
molecule, should present higher conversions [35]. In addition, it
can be observed that the improvement of the conversion for the
dealuminated zeolites depends on the conditions of the treatment,
higher conversions being obtained for the HZ/1.0/60 and HZ/2.0/60
samples than for the HZ/0.5/60 and HZ/1.0/80 zeolites.
Therefore, through the analysis of the heptane and methylcyclo-
hexane results, it can be concluded that HZ/1.0/60 and HZ/2.0/60
dealuminated zeolites appear to be the samples with the most
appropriated properties in terms of acidity and accessibility.
Table 2
Evaluation of Brønsted and Lewis (overall) acidity and external acidity of the different catalysts.
Catalyst Total acidity (lmol gÀ1
) External acidity (lmol gÀ1
)
[B]a
(lmol gÀ1
) [L]b
(lmol gÀ1
) [B]/[L] [B]450/[B]150 [L]450/[L]150
HZ 351 236 1.48 0.13 0.49 34
HZ/0.5/60 275 232 1.18 0.19 0.52 –
HZ/1.0/60 327 194 1.68 0.21 0.57 44
HZ/1.0/80 243 152 1.60 0.16 0.53 40
HZ/2.0/60 298 256 1.17 0.21 0.51 44
a
[B] concentration of Brønsted acid sites.
b
[L] concentration of Lewis acid sites.
30 32 34 36 38 40 42 44 46 48 50
66
68
70
72
74
76
78
80
HZ/1.0/60
HZ/2.0/60
HZ/1.0/80
HZ
Externalsurfacearea(m
2
g
-1
)
External acidity (µmol g
-1
)
Fig. 5. External surface area obtained from N2 adsorption isotherm as a function of
external acidity determined by collidine adsorption.
S.S. Vieira et al. / Microporous and Mesoporous Materials 201 (2015) 160–168 165

7. 3.3. Oleic acid esterification
Fig. 8 presents the results obtained for oleic acid conversion into
methyl oleate with the parent and dealuminated zeolites, for dif-
ferent oleic acid/methanol ratios. It is important to refer that some
catalytic tests were also carried out in absence of catalyst, under
the same operating conditions, in order to evaluate the extent of
the thermal conversion and, so, the contribution of the catalyst
for the final conversion. Actually, it is claimed that temperature
alone can have a great effect on the conversion of the oleic acid
[18]. The conversions obtained without catalysts were about 34%,
35% and 27% for the 1:45, 1:20 and 1:10 M ratios, respectively,
meaning that, effectively, the temperature can significantly influ-
ence the results. This behavior may be explained by the presence
of Brønsted acidity in the fatty acids.
Analyzing the results found in presence of catalysts, it can be
seen that the parent HZSM-5 was active for the conversion of the
oleic acid into methyl oleate and that the conversions attained
were higher than those obtained in absence of catalyst, confirming
the beneficial effect of using a catalyst. Moreover, it can be noticed
that, whatever the acid oleic/methanol molar ratio, there is a
further improvement of the oleic acid conversion when using
0
5
10
15
20
25
Conversionofheptane(%)
HZ/2.0/60HZ/1.0/80HZ/1.0/60HZ/0.5/60HZ
(a)
100 150 200 250 300 350
0
4
8
12
16
20
24
28
Conversionofheptane(%)
Brønstedacidsites (µmol g
-1
)
HZ/1.0/80 HZ/0.5/60
HZ/1.0/60
HZ
HZ/2.0/60
(b)
Fig. 6. (a) Heptane initial conversion (2 min TOS) for the parent and dealuminated zeolites and (b) evolution of the heptane initial conversion with the number of Brønsted.
0
2
4
6
8
10
HZ/2.0/60HZ/1.0/80HZ/1.0/60HZ/0.5/60HZ
Conversionofmethylcyclohexane(%)
(a)
30 35 40 45 50
0
2
4
6
8
10
HZ/1.0/60
HZ/2.0/60
HZ/1.0/80
HZ
External acidity (μmol g
-1
)
Conversionofmethylcyclohexane(%)
(b)
Fig. 7. (a) Methylcyclohexane initial conversion (2 min TOS) for the parent and dealuminated zeolites and (b) evolution of the methylcyclohexane initial conversion with
external acidity.
Fig. 8. Oleic acid conversions obtained for the different oleic acid/methanol ratios.
166 S.S. Vieira et al. / Microporous and Mesoporous Materials 201 (2015) 160–168

8. dealuminated zeolites, which is dependent on the dealumination
degree. The maximum oleic acid conversions were obtained for
the HZ/1.0/60 zeolite. This clearly shows that the zeolites when
treated with citric acid can lead to better results for the esterifica-
tion reaction.
The increase of the oleic acid conversion with the dealumina-
tion can be a result of several effects. According to Chung and Park
[19], who studied the oleic acid esterification on different acid zeo-
lites (HMFI, HMOR, HFAU and HBEA), both the quantity and
strength of acid sites can have an important role in the oleic acid
conversion, the latter parameter being the most relevant. For the
dealuminated zeolites of the present study, it was observed a
decrease of the total number of Brønsted acid sites, but this was
accompanied by an enhancement of their acid strength (Table 2),
which could, so, result in an increase of the conversion.
Nevertheless, the improved acidity strength should only explain
in part the better oleic acid conversions obtained with the treated
zeolites. In fact, in this analysis, it is also important to consider that
the oleic acid molecule presents a kinetic diameter (5.5 Å, Gaussian
09 program) that is very close to the size of the HZSM-5 pore open-
ing (5.4 Â 5.6 Å–5.1 Â 5.4 Å). This means that most of the oleic acid
molecules are not able to enter and diffuse within the HZSM-5 zeo-
lite channels, even more at low reaction temperature. Therefore,
the reaction should mainly take place on the outer surface of the
zeolites. Consequently, the evolution of oleic acid conversion with
the external surface area and the total external acidity was evalu-
ated (Fig. 9). It can be seen that, effectively, the higher these
parameters, the greater the conversion values obtained, confirming
that, for this particular zeolite, the oleic acid conversion is mainly
governed by the enhancement of the zeolite external surface prop-
erties. Actually, the treatment with citric acid, can lead to a higher
external surface and, so, to an increase of the number of acid sites
accessible to oleic acid molecules.
Hence, globally, the best oleic acid conversion results were
obtained for both the HZ/1.0/60 and HZ/2.0/60 dealuminated sam-
ples, as they registered the smallest loss of Brønsted acid sites and
the highest increase of the acidity strength, external surface area
and external acidity. The improved characteristics of these two
zeolites were also confirmed by methylcyclohexane and heptane
transformations data.
Another important remark is that the increase of the catalysts
efficiency with the dealumination treatment seems to be even
more significant at higher acid oleic/methanol molar ratios
(Fig. 8). For the parent HZSM-5 zeolite, a decrease of about 37%
in conversion can be observed with the increase of the acid oleic/
methanol molar ratios to 1:45 from 1:10, whereas, for the dealumi-
nated samples, only 2–20% of reduction was noticed. Indeed, the
enhanced molecules diffusion and the higher number of molecules
reaching the acid sites on the dealuminated samples could enhance
the esterification reaction kinetics, without being necessary to use
higher methanol amounts to improve thermodynamics. Thus, the
possibility to operate with lower quantities of methanol using dea-
luminated zeolites could be important at an industrial point of
view, since it would allow using smaller size equipment and lower
energy inputs. Table 3 presents studies using zeolites as catalysts
for biodiesel production via esterification.
4. Conclusion
The results indicated that mild dealumination treatments
performed on HZSM-5 zeolites with citric acid are beneficial for
68 70 72 74 76 78 80
50
55
60
65
70
75
80
85
90
HZ/2.0/60
HZ/0.5/60 HZ/1.0/80
HZ/1.0/60
HZ
External area (m
2
g
-1
)
Conversionofoleicacid(%)
(a)
30 32 34 36 38 40 42 44 46 48 50
50
55
60
65
70
75
80
85
90
HZ/1.0/60
HZ/2.0/60
HZ/1.0/80
External acidity (µmol g-1
)
HZ
Conversionofoleicacid(%)
(b)
Fig. 9. Evolution of the oleic acid conversion with (a) external surface area and (b) external acidity.
Table 3
Comparison of maximum esterification conversions using zeolites and those obtained in this study.
Catalysts Reaction conditions Conversion (%) References
Temperature (°C) Catalysts (%) Molar ratio Time (h)
HZ 100 10 1:45 4 55 Present study
HZ/1.0/60 100 10 1:45 4 83 Present study
HZSM-5 100 10 1:45 7 80 [16]
La2O3/SO4
2À
/HZSM-5 100 10 1:45 7 100 [16]
HBEA 130 2 1:3 1 35 [18]
HMOR 130 2 1:3 1 50 [18]
HZSM-5 130 2 1:3 1 40 [18]
HY 130 2 1:3 1 35 [18]
HMFI 60 5.5 1:15 1 80 [19]
HMOR 60 5.5 1:15 1 80 [19]
HFAU 60 5.5 1:15 1 75 [19]
HBEA 60 5.5 1:15 1 70 [19]
USY 200 10 1:6 2 73.9 [52]
WO3/USY 200 10 1:6 2 80 [52]
S.S. Vieira et al. / Microporous and Mesoporous Materials 201 (2015) 160–168 167

9. esterification reaction of oleic acid into methyl oleate. The materi-
als modified with citric acid treatments, using different concentra-
tions and temperatures, evidenced as slight increase of external
surface areas and a reduction of both Brønsted and Lewis acid sites
which is in line with the increase of the Si/Al ratio from 11 to 15.
The samples treated at 60 °C presented the highest Brønsted acid-
ity strength and the greatest improvement of external surface area
which explains the increase of external acidity measured by
collidine adsorption followed by FTIR. These samples present the
most appropriated properties in terms of acidity and accessibility
for promoting the oleic acid conversion that appears to be mainly
governed by the enhancement of the zeolite external surface
properties. The results also showed that whatever the acid oleic/
methanol molar ratio, there is a further improvement of the oleic
acid conversion when using dealuminated zeolites.
Acknowledgment
The authors thank CAPES for granting the scholarship, FAPEMIG
and CNPq for financial support, LGRQ/UFLA for chromatographic
analyses, Instituto Superior Técnico (IST, Lisbon) for materials
characterization, in particular Prof. Teresa Duarte for access to
the XRD facility (CONC-REEQ/670/2001) and Zeochem by HZSM-
5 sample.
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