Effect of the properties of MFI zeolites on the esterification of isopentyl acetate

Article abstract of DOI:10.1016/j.cattod.2020.07.010

Fusel oil is a by-product from the fermentation of sugar to produce ethanol. With the increase in ethanol production in the last years, thousands of liters of fusel oil are also generated. Thus, alternatives for reusing fusel oil involving the transformation of isopentanol, its main constituent, into products relevant to industry have aroused interest. In this work, the use of MFI zeolites with different contents of aluminum in the transformation of isopentyl into its ester, isopentyl acetate, was studied. Its main objective was to investigate the effects of the aluminum content of zeolites on textural and chemical properties and how they influence the catalytic activity. In the work it was verified that the catalysts with larger external surface obtained better catalytic performance, suggesting that, due to the small pore diameter of the MFI structure, the reaction is limited by the effects of internal diffusion and occurs mainly at the external surface of the MFI particles.

Full text of DOI:10.1016/j.cattod.2020.07.010

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Effect of the properties of MFI zeolites on the esterification of isopentyl
acetate
Maria E. Ribeiro (Investigation) (Formal analysis) (Writing - review
and editing), Karen N. Franke (Formal analysis) (Writing - review
and editing), Patr´ıcia M. Lima (Conceptualization) (Writing - review
and editing), Dilson Cardoso (Conceptualization) (Supervision)
(Writing - review and editing) (Funding acquisition)
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DOI:
https://doi.org/10.1016/j.cattod.2020.07.010
CATTOD 13002
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Catalysis Today
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17 July 2020
Please cite this article as: Ribeiro ME, Franke KN, Lima PM, Cardoso D, Effect of the
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doi: https://doi.org/10.1016/j.cattod.2020.07.010
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Effect of the properties of MFI zeolites on the esterification of isopentyl acetate
Maria E. Ribeiro, Karen N. Franke, Patrícia M. Lima, Dilson Cardoso*
Catalysis Laboratory - Department of Chemical Engineering,
Federal University of São Carlos, 13565-905, São Carlos, SP, Brazil
*Corresponding author: dilson@ufscar.br
Graphical abstract
Highlights
 MFI zeolites with different aluminum contents were used to obtain isopentyl acetate
 Catalysts MFI with larger external surface obtained better catalytic performance
 The esterification over MFI occurs at the external surface of the particles
 The esterification reaction is limited by the internal diffusion
Abstract
Fusel oil is a by-product from the fermentation of sugar to produce ethanol. With the increase
in ethanolproduction in the last years, thousands of liters of fusel oil are also generated. Thus,
alternatives for reusing fusel oil involving the transformation of isopentanol, its main
constituent, into products relevant to industry have aroused interest. In this work, the use of
MFI zeolites with different contents of aluminum in the transformation of isopentyl into its
ester, isopentyl acetate, was studied. Its main objective was to investigate the effects of the
aluminum content of zeolites on textural and chemical properties and how they influence the

catalytic activity. In the work it was verified that the catalysts with larger external surface
obtained better catalytic performance, suggesting that, due to the small pore diameter of the
MFI structure, the reaction is limited by the effects of internal diffusion and occurs mainly at
the external surface of the MFI particles.
Keywords: Esterification, MFI zeolite, shape selectivity, diffusional limitation, isopentyl
acetate
1. Introduction
The alcohol industry generates several by-products and among these, fusel oil
stands out, due to the high volume generated (65 million liters/year in Brazil) [1,2] and its
composition, rich in higher alcohols such as isopentanol, isobutanol, butanol, and propanol
[3,4]. Isopentanol is the main constituent of this by-product and can be used industrially to
some products used in industry [3]. However, due to the high production, alternative uses
of this by-product are essential to make the alcohol industries more profitable [4].
Esters are one of the main classes of organic compounds, used as flavorings in food,
beverages, cosmetics and in the pharmaceutical industry [5]. Among the esters of
isopentyl alcohol, isopentyl acetate stands out, one of the most used flavorings in the food
industry with worldwide demand of approximately (74,000 kg/year), due to the strong
aroma of fruits [1,6]. Furthermore, it can be used as a solvent, varnishes, silk, and films
[7] .
Typically, esterification reactions can be performed using homogeneous acid
catalysts, such as H2SO4 [8]. However, heterogeneous acid catalysts offer advantages in
easing the separation of reaction products and the absence of equipment corrosion [9].
Thus, zeolites [8,10–12], ion exchange resins [7,8,13], sulfonated zirconia [8], niobium

oxides [8,14], among others, have been reported in the literature as potential catalysts for
esterification reactions [9].
According to the literature, ion exchange resins were the first heterogeneous
catalysts used in the esterification reaction of acetic acid with short-chain alcohols [8,13].
Although they have a high catalytic activity, resins have low thermal resistance, limiting
their use to 120 °C and for short periods [8]. In addition to resins, niobium oxides have
been shown to be efficient for esterification due to the high surface acidity [8,14].
Protonic zeolites such as Y, BEA and MFI also provide an interesting alternative in which
the performance of these catalysts is based mainly on strong acidity due to the presence
of Brønsted sites [8]. However, several parameters are mentioned in the literature that
influences the catalytic performance of these materials, such as diffusional limitation in
the pores, the type of channels (uni-bi and three-dimensional) and hydrophobicity [12].
In this work, acidic zeolites of MFI structure with different percentages of
framework aluminum were applied as catalysts in the esterification of acetic acid to obtain
isopentyl acetate. The objective is to investigate the effects of aluminum content on the
textural, chemical and catalytic properties of zeolites.
2. Materials and methods
2.1. Preparation of catalysts
The ammoniacal zeolites with MFI structure (CBV 2314, CBV 3024E, CBV 2524G
and CBV 8014) were purchased from Zeolyst International (USA). To obtain the
protonated form, these materials were calcined in a muffle at a temperature of 500 ° C for
4 h, in a static atmosphere, using a heating ramp of 10 °C. min-1.
The sodium zeolite MFI (SN-55) was acquired from ALSI-PENTA Zeolithe
Gmbh and its acid form was achieved through ion exchange with aqueous NH4Cl solution

(1 mol. L-1). For this procedure, the sodium zeolite was subjected to three consecutive ion
exchanges of 1 h; afterward, the material was washed with distilled water and dried at
80 °C for 24 h. After the ion exchange step, the material was calcined under the same
conditions as the other ammoniacal samples. The nomenclature and some properties of
the catalysts are shown in Table 1 and it shows almost all aluminum present in these
zeolites are in the framework, as determined by 27Al MAS-NMR [15].
Table 1. Nomenclature of commercial and acidic zeolites, their aluminum (% Al) and
acidic sites content (NAS).
Product
Code
Catalyst
Codea
c
d
Si/Al
% Alb
Alf
nd
NAS
7.7
6.9
5.4
4.3
2.5
1.3
CBV 2314
CBV 3024E
SN-55
11.5
15
HZ-8
HZ-7
HZ-5
HZ-4
HZ-3
0.91
nd
1.0
0.90
0.67
0.41
22
0.94
0.97
CBV 2524G
CBV 8014
25
40
aHZ= protonated zeolite.
b %Al= Molar fraction of aluminum estimated by EDS: 100*Al/ (Al+Si).
c Alf= Framework aluminum molar fraction =Alf/ (Alf+Alef), [15].
d NAS = acid sites mmols per catalyst gram.
EDS= Energy Dispersive X-Ray Spectroscopy.
nd = not determined
2.2.
Characterizations
The confirmation of the structure of these zeolites was performed using a
diffractometer (Miniflex 600, Rigaku) with Cu-Kα radiation (40 kV, 15 mA), varying 2θ
in the range of 5 to 50 °.

The Si and Al content of the samples was determined by EDS; the samples were
dispersed on double-sided carbon adhesive tapes and placed on sample holders. The analyses
were performed in an XL FEG electron microscope, operated at a voltage of 20 kV.
The protonated catalysts were analyzed by scanning electron microscopy (SEM)
to verify the shape and size of the particles. The sample suspensions were prepared by
adding 20 mg of catalyst in methanol, performing the dispersion by means of ultrasound
for 30 min. Subsequently, aliquots of the supernatant were dripped onto a sample holder
until the deposition of the solid and complete evaporation of the solvent was observed.
The micrographs were acquired by an XL FEG electron microscope, operated at a voltage
of 5 kV.
The isotherms were carried out with a Micromeritics ASAP 2020 equipment.
Initially, the samples were treated under vacuum, to eliminate physical water in the
catalyst. After treatment, the volume of N2 adsorbed at low pressures and at the boiling
temperature of liquid nitrogen (-195.8 °C) was measured. The volume of micropores and
the external surface area of the catalysts were calculated by analysis of nitrogen
physisorption isotherms by the t-plot method.
The total amount of acidic sites in the zeolites was determined by ammonia
desorption at programmed temperature (TPD-NH3), using an AutoChem II 2920
chemisorption analyzer from Micromeritics.
2.3. Catalytic Tests

The catalytic activity of protonated zeolites was evaluated in the reaction of
esterification of isopentanol with acetic acid whose mechanism is well known [7]. The
esterification reaction was conducted in the proportion of acetic acid and isopentyl
alcohol, as shown in Figure 1.
Figure 1. Scheme of the esterification between acetic acid and isopentanol.
The catalytic tests were carried out at 80 °C in a batch reactor equipped with a
condenser. The initial reaction mixture (20 ml) contained equimolar amounts (0.12 mol) of
the reagents (Sigma-Aldrich, 99%) and 4 wt% of catalyst in relation to mass of the reaction
mixture. The reaction components were analyzed in a Shimadzu CG-2010 chromatograph
containing an RTX-1 capillary column and a flame ionization detector (FID). In order to
verify reproducibility, the catalytic tests were carried out in triplicate.
The conversion of acetic acid assumes that it is selectively consumed as shown in
Figure 1, and was calculated using Equation 1, in which MA and MP are the number of
moles of acetic acid and of the ester, respectively.
MP
( )
XA % =
× 100
(Eq. 1)
MA+MP
The kinetic curves obtained from the catalytic tests were adjusted by a hyperbolic
model shown in Equation 2, where t is reaction time, a and b are specific parameters for
each experimental reaction condition.

푎.푡
XA =
(Eq. 2)
(푡+푏)
The adjustment of the curves to this model allowed the determination of the
conversion derivative of acetic acid at the beginning of the reaction, dXA/dt|t=0 . This
parameter allows to estimate the reaction rate at the initial moment of the reaction, 푟퐴|푡=0
by Equation 3, in which CA0 (mol. L-1) is the initial concentration of acetic acid in the
reaction mixture.
(
)
푟퐴|푡=0 = CA0 dXA/dt|t=0
On the other hand, the turnover frequency at time zero, TOF0 (min-1), corresponds
(mol. L-1. min-1), and
(Eq. 3)
to the quotient between the reaction rate at the initial instant, 푟
퐴|푡=0
the number of mols of acidic sites present in the reaction medium N´AS (mol H+).
TOF0 can be calculated from Equation 4, where V(L) is the volume of the reaction
mixture, NAS (mol H+. g-1) is the number of acidic sites in a gram of catalyst and mcat is
the mass of catalyst in the reaction mixture.
r
. V
(
. V . dX /dt|
A t=0
)
C
A0
A|t=0
TOF0 =
=
(Eq. 4)
′
푁
N . m
AS cat
퐴푆
Finally, NAS was calculated by Equation 5, based on the chemical composition of
zeolite, where TCU (equal to 96) is the total number of silicon and aluminum tetrahedra
+
in the unit cell. In this equation, xH is equal to the molar fraction of aluminum in the
+
+
zeolite network, Alf (Table 1); 푀푀퐻 is the molecular weight of H ; x(i)represents the

molar fraction of Si or Al; MM(i) is the molecular mass of Si or Al; MM(O2) is the
molecular mass of oxygen:
TCU . x
. 푀푀
x
. 푀푀
+ +
퐻
+
+
H
퐻
H
NAS
=
=
(Eq. 5)
∑
∑
x
TCU . x ×MM +2 TCU .MM
.MM +2 . MM
(i) (i) (O2)
(i)
(i)
(O2)
Through equation 5, it was obtained that the 푁퐴푆 of the catalysts HZ-8, HZ-7,
HZ-5, HZ-4 and HZ-3 are 1.3, 1.1, 0.9, 0.67 and 0.41 mmol H+/g of zeolite, respectively.
3. Results and Discussions
3.1. Characterization of catalysts
The result of the chemical analysis of zeolites in the protonic form is shown in Table 1.
The aluminum content, % Al, of the zeolites determined by EDS is in accordance with the
information reported by the manufacturers. The EDS technique showed that none of the
samples presented Na+ cations, revealing that the ion exchange achieved was 100%
effective.

Figure 2 shows the X-ray diffractograms of the catalysts in the protonic form. All
diffractograms showed the same profile with more intense peaks at 2θ angles located at
7.9; 8.9; 23.1 and 23.4 ° which are characteristic of zeolites with MFI structure according
to the literature [16]. In addition, the presence of other zeolitic phases was not verified in
the diffractograms.
Figure 2. X-ray diffractograms of protonic zeolites with MFI structure.

The scanning electron microscopy images of the MFI catalysts with different
aluminum contents are shown in Figure 3. The micrographs of the samples HZ-8, HZ-7,
HZ-4 and HZ-3 (Fig. 3a, 3b, 3d, and 3e) revealed that they are formed by particles with
irregular morphology and constituted by polycrystalline agglomerates with non-uniform
size distribution. The micrograph of the HZ-5 zeolite (Fig. 3c), however, showed that it
is formed by particles with well-defined morphology, in the form of hexagonal prisms,
which is characteristic of the MFI materials [17]. In addition, it was found in Fig. 3c, that
the HZ-5 particle size is much larger than the other samples. Other authors also found
that the particle size and morphology of MFI materials depend on the aluminum content
present in the structure [18].

Figure 3. Micrographs of MFI acid zeolites (a) HZ-8, (b) HZ-7, (c) HZ-5, (d) HZ-4 and
(e) HZ-3.
The nitrogen physisorption isotherms of the samples are presented in Figure 4,
and their textural properties are shown in Table 2. According to IUPAC [19], samples
HZ-8, HZ-7, HZ-5, HZ-4, and HZ-3 exhibited type I BET isotherms, possessing high
adsorption capacity at low relative pressures, characteristics of materials with micropores.
Figure 4. Nitrogen adsorption and desorption physisorption isotherms of samples HZ-8,
HZ-7, HZ-5, HZ-4 and HZ-3. Open symbols show desorption.
When applying the t-plot method to these isotherms (Figure 5), it is observed a
linearity between the volume and the thickness of nitrogen physisorbed, in the region of
P/P0 between 0.1 and 0.4 (N2 thickness between 0.36 and 0.49 nm). This allows estimating

the volume of micropores and the external surface of these zeolites, which are shown in
Table 2.
Figure 5. Analysis by t-plot of nitrogen adsorption isotherms of samples HZ-8, HZ-7,
HZ-5, HZ-4 and HZ-3.
Analyzing the values of the textural properties of these zeolites, shown in Table 2,
it can be observed that there is a tendency for the external area of these materials to
increase with the reduction of their aluminum content. For example, the external area
(Sext) of the HZ-8 sample is 75.1 m2/g while HZ-3 was much higher: 169.7 m2/g. The
increase in the external area means that the size of the particles decreases as the aluminum
content decreases, which agrees with other authors [18]. The sample HZ-5 is an
exception, which, different to the samples manufactured by Zeolyst, has a well-defined
habit and much larger particles (Fig. 3c), certainly due to very different synthesis
conditions.

Table 2. Textural and catalytic properties of protonic MFI zeolites.
a
a
b
c
Vmicro
Sext
dXA/dt|t=0
(min-1)
0.082
TOF0
NAS
Sample
(m2/g)
(min-1)
0.107
0.229
0.132
0.527
0.829
(cm3/g)
HZ-8
HZ-7
HZ-5
HZ-4
HZ-3
1.3
1.0
0.13
75.1
0.10
80.1
0.141
0.90 0.13
0.67 0.12
0.41 0.11
58.1
0.071
99.7
0.210
169.7
0.201
a Determined by the t-plot of the N2 adsorption isotherms.
b Determined by the derivative (for t= 0) of the function adjusted (Eq. 2) to the experimental points
(R2= 0.99).
c TOF0 = turnover frequency at time zero determined by (Eq. 4).
Figure 6 shows the ammonia programmed temperature desorption curves (NH3
TPD) resulted from the acidic zeolites HZ-8, HZ-7 HZ-5, HZ-4 and HZ-3. The ammonia
desorption profiles showed two peaks, the first (A) being around 195-217 ° C and the
second (B) between 360-400 ° C. The peak at the lowest temperature is attributed to the
desorption of ammonia adsorbed on the ammonium cation [20,21] while the peak at the
highest temperature can be explained by the decomposition of the ammonium cation,
resulted from adsorption of ammonia to strong acid sites [21].

The ammonia TPD curve of the HZ-5 zeolite has a similar profile to the others, but
its maximum desorption peaks A and B are shifted to 20 and 40º C higher temperatures,
respectively. To explain this behavior, the micrographs shown in Figure 3c show that this
zeolite, unlike the others, has well-formed crystals and of much larger dimensions. Zhao
and collaborators [22] found the same effect, of displacement to higher temperatures with
increasing particle sizes of their microporous solid. This behavior can be explained by a
“labyrinth effect” in which, when dimensions are much larger, the desorbed ammonia
molecule takes longer to leave the particle. Consequently, the identification of desorbed
ammonia in the gas phase is displaced at higher temperatures.
Figure 6. TPD-NH3 profile of MFI proton zeolites with different framework aluminum
content.
Table 3 shows the amount of ammonia desorbed from the zeolites, obtained through
the deconvolution of the TPD-NH3 curves using Gaussian functions (Figure 6), resulting in

adjustments greater than 0.97. The results show that the amount of desorbed ammonia
decreases with the reduction of the framework aluminum content in the zeolite, Alf, shown
in Table 1 [15]. This confirms that most aluminum atoms are found in the zeolitic network,
thus generating Brønsted acidic sites. In addition, it is observed that the samples HZ-8, HZ-7,
HZ-4 and HZ-3 showed NH3 desorption peak B occurs in the same temperature range, around
360° C (Figure 6), confirming that the acid sites have the same strength, as predicted by
Barthomeuf for high Si/Al ratio zeolites [23]. The Table 3 also shows the ratio between the
amount ofammonia desorbed at peak A in relation to peak B, with values lower than or equal
to 1.0, reinforcing the hypothesis that the first peak is due to the desorption of ammonia that
is adsorbed on the cation ammonium, this acting as a Lewis site.
Table 3. Quantification of acidity by NH3-TPD and the relationship between desorption
peaks A and B.
Peak A NH3
(°C)
Peak B
NH3 peak Total NH3 NH3 peaks
Catalysts
peak Aa (°C)
Ba
desorbed a A/B ratio
HZ-8
HZ-7
HZ-5
HZ-4
HZ-3
203
695
462
494
277
193
363
358
398
362
352
815
1,510
1,004
988
0.85
0.85
1.0
200
217
195
196
542
494
399
301
676
0.69
0.64
494
Peak A= First peak, Peak B= Second peak.
a μmol/g.
3.2.Catalytic Tests

Figure 7 shows the experimental results of the conversions, during two hours of
reaction, of acetic acid in isopentyl acetate calculated by Equation 1. The figure also
shows the curves adjusted to the experimental data, using Equation 2. Previous works
show that the esterification increases with the number of acidic sites [8,11]. However, the
results in this Figure 7 show, for example, that the HZ-4 catalyst showed superior catalytic
performance to the others, although it has one of the lowest contents of catalytic sites,
NAS (Table 1). Similarly, HZ-8 has the highest aluminum content among the other
samples but has a very small activity. This suggests that there are other factors that may
be influencing catalytic activity.
Figure 7. Conversion of acetic acid to isopentyl acetate using proton catalysts with MFI
structure.
Analyzing the chemical and catalytic properties of the catalysts presented in
Table 2 it is observed that, in general, as the number of catalytic sites (NAS) decreases,

there was an increase in the turnover frequency of the esterification, TOF0. This result is
not expected since, in this range of aluminum content (Table 1), the strength of acidic
sites in zeolites is constant [23] and, therefore, TOF0 should not vary.
Table 2 also shows that as the aluminum content in these zeolites decreased, there
is a tendency for their specific external area to increase. Therefore, it can be assumed that,
as found by Viswanadham et al [24], the catalytic activity is related to the external surface
of the catalysts. This is evident in Figure 8, where there is a pronounced increase in the
turnover frequency, TOF0, with the specific external area of most zeolites. This behavior
may be a consequence of limitations of the internal diffusion by some of the reaction
components, probably isopentyl alcohol, known as reactant shape-selectivity. Another
possibility to explain this behavior could be a consequence of the transition state shape-
selectivity. In both types of shape-selectivity, the consequence would be the same, that
is, the catalytic action would be occurring preferably on the acid sites present at the outer
surface of the particles of these zeolites [25].
This hypothesis is supported by the results presented in Figure 8, in which it is
observed that TOF0 increases with the value of the external area of the zeolites studied.
The HZ-5 zeolite does not follow this behavior, but its morphology is completely different
from the other samples, as seen in the discussion of the micrograph presented in Figure 3c.

Figure 8. Turnover frequency TOF0 of the esterification as a function of the external
surface area of catalysts HZ-8, HZ-7, HZ-5, HZ-4 and HZ-3.

4. Conclusions
The zeolites with MFI structure with different contents of aluminum are active for
esterification of acetic acid with isopentyl alcohol under mild reaction conditions.
Contrary to what found by some authors in other zeolites, the increase in aluminum
content in the MFI zeolites did not always increase the catalytic activity, estimated by the
frequency of molecules transformed per site (TOF0). According to the results, this
unexpected behavior can be attributed to the reduction of the external area of the catalysts
with the increase of the aluminum content in the MFI zeolites. This suggests that, due to
the diffusional limitation of some component of the reaction in the micropores of the MFI
(probably isopentanol or isopentyl acetate), the reaction occurs preferentially on the
external surface of the catalyst. Therefore, in defining the catalytic activity for this
esterification reaction there are two parameters to be considered: the number of acidic
sites and the value of the external surface of the catalyst. According to the results of this
work, the main factor that increases the catalytic activity in the formation of isopentyl
acetate is the number of acid sites present at the external surface of the catalyst.
CRediT author statement
Maria E. Ribeiro, Investigation, Analysis, Writing, Review
Karen N. Franke, Analysis, Review
Patrícia M. Lima, Conceptualization, Review
Dilson Cardoso, Conceptualization, Supervision, Review, Funding
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Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
– Brasil (CAPES). The authors thank the Laboratory of Structural Characterization
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