Zr-modified hierarchical mordenite as heterogeneous catalyst for glycerol esterification

Article abstract of DOI:10.1016/j.catcom.2017.06.009

Hierarchical mordenite catalysts were prepared by HF and NH₄F etching. Materials' zirconia modification was performed by incipient wetness impregnation in order to vary surface acidity type and strength. XRD, N₂ physisorption, solid-state NMR and UV–Vis spectroscopies were applied for characterization. Acidic properties of the obtained materials were investigated by TPD of adsorbed ammonia and FT-IR spectra of adsorbed pyridine. Catalytic performance was studied for the glycerol esterification with acetic acid. Zr increased reaction turnover activity compared to raw zeolite substrate due to abundant Br?nsted and Lewis moiety presence. The demonstrated biomass valorization processes to bio-based platform chemicals are of interest, as they employ biodiesel production waste.

Full text of DOI:10.1016/j.catcom.2017.06.009

Accepted Manuscript
Zr-modified hierarchical mordenite as heterogeneous catalyst for
glycerol esterification
Margarita Popova, Hristina Lazarova, Yuri Kalvachev, Totka
Todorova, Agnes Szegedi, Pavletta Shestakova, Gregor Mali,
Venkata D.B.C. Dasireddy, Blaž Likozar
PII:
S1566-7367(17)30240-6
doi: 10.1016/j.catcom.2017.06.009
CATCOM 5075
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
28 February 2017
18 May 2017
12 June 2017
Please cite this article as: Margarita Popova, Hristina Lazarova, Yuri Kalvachev, Totka
Todorova, Agnes Szegedi, Pavletta Shestakova, Gregor Mali, Venkata D.B.C. Dasireddy,
Blaž Likozar , Zr-modified hierarchical mordenite as heterogeneous catalyst for glycerol
esterification. The address for the corresponding author was captured as affiliation for all
authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.06.009
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ACCEPTED MANUSCRIPT
Zr-modified Hierarchical Mordenite as Heterogeneous Catalyst for Glycerol
Esterification
1
*
1
2
2
3
Margarita Popova , Hristina Lazarova , Yuri Kalvachev , Totka Todorova , Agnes Szegedi ,
1
4
4
4
Pavletta Shestakova , Gregor Mali , Venkata D. B. C. Dasireddy , Blaž Likozar
1
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of
Sciences, 1113 Sofia, Bulgaria
2
Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, 1113 Sofia,
Bulgaria
3
Research Centre for Natural Sciences, Institute of Materials and Environmental Chemistry,
Hungarian Academy of Sciences, 1117 Budapest, Hungary
4
National Institute of Chemistry, 1001 Ljubljana, Slovenia
*
Corresponding author: Margarita Popova, Institute of Organic Chemistry with Centre of
Phytochemistry, Acad. G. Bonchev str. Bl. 9, 1113 Sofia, Bulgaria, Tel.: +359 2 9793 961;
Fax: +359 2 8700 225; E-mail: mpopova@orgchm.bas.bg

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Abstract
Hierarchical mordenite catalysts were prepared by HF and NH4F etching. Materials’ zirconia
modification was performed by incipient wetness impregnation in order to vary surface
acidity type and strength. XRD, N2 physisorption, solid-state NMR and UV-Vis
spectroscopies were applied for characterization. Acidic properties of the obtained materials
were investigated by TPD of adsorbed ammonia and FT-IR spectra of adsorbed pyridine.
Catalytic performance was studied for the glycerol esterification with acetic acid. Zr increased
reaction turnover activity compared to raw zeolite substrate due to abundant Brönsted and
Lewis moiety presence. The demonstrated biomass valorization processes to bio-based
platform chemicals are of interest, as they employ biodiesel production waste.
Keywords: renewable glycerol esters, biodiesel production waste, hierarchical zeolite
materials, bio-based chemicals conversion, chemical reaction engineering

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1
. INTRODUCTION
The increasing demand for energy, chemicals and materials in our society has led research
activity towards the development of green technologies, based on renewable resources. Food
waste is one of those renewable sources of carbon. Biofuels are an attractive alternative to
fossil fuels due to their positive influence on the environment and the fact that they are
produced from renewable sources. The production of biodiesel on an industrial scale is
achieved through homogeneously catalyzed esterification. The use of refined oil and fats (first
generation raw materials) has led to a high cost of the end product and contradiction
food/fuels. The production of biofuels on the basis of waste has given research a new spur due
to the low cost of the initial raw material, as well as the desire to utilize waste and solve the
economic, environmental and social problems. Used cooking oil (UCO) is currently one of the
most attractive feedstocks for the production of biodiesel [1-3], mostly due to the low market
value of the feedstock. Glycerol is generated as a byproduct in conventional biodiesel
production and can be converted to various valuable chemicals through numerous routes such
as etherification [6], hydrogenolysis [7], oxidation [8], transesterification [9] and dehydration
[10]. Besides, glycerol can be esterified with acetic acid to obtain valuable products of mono-,
di- and triacetyl glycerol (named as MAG, DAG and TAG, respectively), which have found
versatile industrial applications [11]. MAG and DAG have applications in cosmetics,
medicines and as a starting monomer for the production of biodegradable polyesters [1, 11];
whereas TAG has application as a biodiesel additive [12]. Traditionally, glycerol
esterification with acetic acid is performed over conventional acids as homogeneous catalysts
[11]. A major drawback to using homogeneous catalysts is that they lead to serious technical
and environmental problems. Therefore, employing heterogeneous acid catalysts can
contribute to overcome these drawbacks [1, 11, 13–16]. Due to their stronger Brönsted acidity
than conventional solid acids like mixed oxides and zeolites, Keggin type heteropolyacids
(HPAs) can be employed instead of classical homogeneous acids [17-29]. Some drawbacks of
bulk HPAs like low thermal stability, high solubility in water and polar media and low surface
2
−1
area (1–10m g ) can be improved by functionalizing them into the framework of high surface
area porous supports (silica, activated carbon, zirconia, polymers and acidic ion-exchange
resin) [11, 18, 19]. Hierarchical zeolites are good candidates for catalysts and carriers of
catalytic active phase due to their high surface area, adjustable structure, chemical and
thermal stability [30-32].

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Despite the numerous investigations, the problem with the development of a highly acidic
heterogeneous catalyst for catalytic utilization of food waste and the development of green
technologies to obtain valuable chemicals on their basis still exists.
In the present study hierarchical mordenite was prepared by HF and NH4F etching and
modified with ZrO2. The obtained parent, hierarchical and their Zr-analogs were studied in
glycerol esterification with acetic acid. The influence of the textural characteristics of the
hierarchical zeolites and the type of the formed acidic sites on the activity to produce glycerol
esters is discussed.
2
. EXPERIMENTAL
2
.1 Synthesis of parent and hierarchical mordenites
Mordenite was synthesized from a suspension with a molar composition 18SiO2: Al2O3:
.24K2O: 1.21Na2O: 22.5H2O to which are added 5 wt.% mordenite crystals as seed. The
1
crystallization was performed in stainless-steel teflon-lined autoclave under autogenous static
о
conditions for 18 h at 180 C. The obtained highly crystalline mordenite is used for post-
synthetic acid treatments with hydrofluoric acid and ammonium fluoride. After that, the
samples were converted in H-form by treatment with 1 M solution of ammonium nitrate at
о
3
73 K for 24 h and subsequent centrifugation, washing and calcination at 450 C [33].
2
.2 Functionalization of the parent and hierarchical mordenite by ZrO2
An incipient wetness impregnation technique with ZrCl2 (99 %, Aldrich) was applied for
loading of 15 wt. % metal oxide. In a typical experiment 1g of mordenite samples were mixed
with 1 ml aqueous solution of ZrCl2 at room temperature. The precursor salt was decomposed
in air at 500°C for 2 hours. Samples were designated as ZrM (M=parent mordenite) and
Zr/M1 (M1=hierarchical mordenite).
2
.3 Characterization
The X-ray diffraction (XRD) powder patterns were recorded on diffractometer d2 phaser
Bruker) with Cukα radiation, working at acceleration 30 kV and current 10 mA.
(
The scanning electron microscopy (SEM) analyses were carried out on a Philips 515
instrument working at 20 kV accelerating voltage.
Nitrogen adsorption-desorption isotherms were obtained with micromeritics ASAP 2010 gas
analyzer. Micro- and mesopores contributions were determined by the t-plot method. The
surface area was calculated by the BET equation and the mesopore distribution was evaluated
by the BJH method.

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Temperature programmed desorption of ammonia (NH3-TPD) was carried out using a
Micromeritics 2920 Autochem II Chemisorption Analyser. In the NH3-TPD experiments, the
catalyst was pretreated at 500 °C under the stream of helium for 60 min. After that the
temperature was then decreased to 80 °C. A mixture of 9.8% NH3 in He as passed over the
catalyst at a flow rate of 25 mL/min for 60 min. The temperature was then raised gradually to
9
00 °C by ramping at 10 °C /min under the flow of helium. The quantity of the NH3 desorbed
was calculated from a detailed deconvolution of NH3-TPD profile.
All solid-state NMR experiments were carried out on a 600 MHz Varian NMR system
1
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27
equipped with a 3.2 mm Varian HX CPMAS probe. Larmor frequencies for H, Si, and Al
2
7
nuclei were 599.53 MHz, 119.10 MHz, and 156.22 MHz, respectively. For Al magic-angle
spinning (MAS) measurements excitation pulse of 0.8 µs was used, number of scans was
4
000 and repetition delay between scans was 0.5 s. Sample rotation frequency was 12.5 kHz.
2
9
For Si MAS measurements excitation pulse was 2.2 µs, repetition delay was 30 s, and
1
29
number of repetitions was 1000. In the H- Si cross-polarization MAS (CPMAS) experiment,
1
H excitation pulse of 2.3 µs was used.
FT-IR experiments were performed with a Nicolet spectrometer by the self-supported wafer
technique with pyridine (Py) as probe molecule. Self-supported pellets were pressed from the
o
-6
samples, placed into the IR cell, heated up to 350 C in high vacuum (10 mbar) with a rate of
o
o
1
0 C/min and dehydrated for 1h. Following 30 min contact with Py at 100 C the sample was
o
evacuated at 200 C for 30 min.
2
.4. Catalytic experiments
o
Prior to the catalytic experiments the samples were pretreated ex-situ for 1 hour at 140 C in
air. A batch reactor equipped with magnetic stirrer was used to perform the reaction. The
reactor was charged with 2 g glycerol and 0.1 g catalyst, glycerol/acetic acid weight ratio =
o
1
:10. The reactor was heated to the desired reaction temperature (100 C) for 3 h and reflux is
used for condensation of the evaporated compounds. The analysis of the reaction products
was performed using HP-GC equipped with FID.
3
. RESULTS AND DISCUSSION
3
.1. Physico-chemical properties
The mordenite sample with average particle size 4 µm was used as a parent material [33]. The
treatment of zeolites with mixture of HF and NH4F results in the formation of bigger pores in
the initial zeolites [34]. XRD patterns of the parent sample and the sample treated for 20
minutes with 0.1 M solution of HF and NH4F (Fig. 1) exhibit very high degree of

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crystallinity. The treatment does not lead to any distortion of the mordenite structure. The
XRD reflections for the formation of crystalline ZrO2 phase is not registered after
modification with ZrCl2 which is an indication for the formation of finely dispersed ZrO2
phase (Fig. 1).
The SEM micrographs of untreated and treated samples are shown in Fig. 2. It can be seen
that the formation of bigger pores in the treated sample occurs. TEM images are also
indicative of such changes (Supplementary data, SD1). These observations were described in
our previous paper [33].
The textural parameters of the studied samples were determined by the nitrogen physisorption
and the data of the parent sample compared with those treated with a 0.1 M solution of
hydrofluoric acid are listed in Table 1. The isotherms are type I, which is typical for
microporous materials. In contrast to the parent sample in isotherms of treated with solution
HF-NH4F is registered a greater uptake of nitrogen at high partial pressure due to the
additional formed pores. The formed additional pores in this case are two times higher than
those in the parent mordenite (1.6 nm for М1 comparing to 0.9 nm for М). Impregnation of
the initial and hierarchical mordenite with zirconia (15 wt. %) resulted in a small decrease of
surface area and pore volume. Decrease of the pores of hierarchical Zr/M1 is registered but
the pore blocking was avoided by the formation of smaller ZrO2 nanoparticles. Zirconia
nanoparticles are immobilized in the hierarchical zeolite matrix due to the interaction with the
Brönsted acid sites on the surface. In the case of Zr/M the pores are not changed after
modification with ZrO2 nanoparticles most probably because they are deposited on the
external surface of the parent mordenite.
The registered peaks at the UV Vis spectra around 200 nm and in the interval 450-500 nm of
the parent mordenite have not changed after the acid treatment of zeolite (SD2). The higher
intensity of the peak at around 200 nm in the spectra of Zr-modified mordenite is due to the
presence of monodispersed Zr atoms surrounded by four oxygen atoms bounded with silicon
in mordenite matrix. The additional peak registered at 275 nm is attributed to the presence of
nanosized ZrO2 particles [35], which is more intensive for Zr-modified hierarchical
mordenite.
2
7
29
Al and Si NMR spectra were used for characterization of the changes of Si/Al ratio after
2
7
the etching and impregnation procedures. The Al MAS spectra of all samples (SD3) show
two signals. The signal at 55 ppm is characteristic for Al atoms in tetra-coordinated
framework positions, while the signal at around 0 ppm is typical for the octahedral extra-
framework Al sites. The results indicate that the acidic treatment with HF and NH4F, as well

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as the modification with zirconia do not result in significant changes in the ratio of framework
to extra-framework Al, since the ratio of the integral intensities of the respective signals in the
2
7
Al spectra is practically the same before and after the treatment.
2
9
Figure 3 shows the Si MAS NMR spectra of the studied samples, where two partially
overlapping signals are observed with chemical shifts of -102 and -109 ppm. The first signal
4
3
can be assigned to Si(1Al) centers (Q ) and/or Si(1OH) centers (Q ), whereas the second
4
4
signal belongs to Si(0Al) centers (Q ). For all samples the quantity of Q structures is higher
3
3
4
as compared to Q . The analysis of the deconvoluted spectra shows that the Q / Q ratio
remains the same before and after the modification of the samples М and М1 with Zr. The
2
9
quantitative analysis of the Si spectra evidenced that for М and Zr/M samples this ratio is
lower as compared to М1 and Zr/M1 samples, respectively. More detailed information about
1
29
the structural environment of the Si centers was obtained from the H- Si CPMAS spectra
cross-polarization magic angle spinning, CPMAS). This technique enables transfer of
(
1
29
magnetization from highly sensitive H to less sensitive
Si nuclei resulting in selective
2
9
3
2
enhancement of Si signals of nuclei with neighboring protons e.g. Q and Q structures. The
broad low intensity signal in the interval from -90 to -100 ppm observed in the CPMAS
2
spectra of the studied samples revealed the presence of small amount of Q structures
3
[
Si(2OH)] in addition to the already detected strong signal for the Q sites.
3
.2. Characterization of the acidic properties
by TPD of adsorbed ammonia
-
TPD of ammonia (Table 1) reveal the presence of the weak, strong and moderate acidic sites
on the surface of catalysts which were evidenced by the three desorption peaks: 150–300 °C
for weak, 300–600 °C for moderate, and above 600°C for strong acidic sites, respectively.
The experiments show that the modification with Zr of hierarchical mordenite slightly
increases the amount of strong acid sites from which the ammonia desorption is registered at
o
1
75-510 C (Table 1). These results together with NMR data are indicative for the additional
formation of strong Lewis acid sites.
by FT-IR spectra of adsorbed pyridine
-
Initial and Zr-modified mordenites show acidic properties, i.e. Lewis and Brönsted acid
surface sites can be distinguished (Fig. 4). The relative amount and nature of these types of
acidic species can be determined by FT-IR spectroscopic investigation of adsorbed pyridine
-
1
-1
(
Py). The bands at about 1610 cm , and 1448 cm are assigned to Py coordinated to Lewis
acid sites (Py-L) (Fig. 4). The protonated Py coordinated to the conjugated base of the solid

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-
1
-1
Brönsted acid (Py-B) gives bands at 1540 cm and 1640 cm . The coordinatively unsaturated
Zr ions on the surface of zirconia can act as Lewis acid sites. Upon the Zr-modification the
intensity of bands characteristic for Lewis acid sites increased, and a small decrease in the
Brönsted acid sites of mordenites is registered. The Brönsted/Lewis acid sites ratio for the
studied samples is presented in Table 2. The calculated value for Zr/M1 sample (0.7) is an
indication for presence of similar amount of Brönsted and Lewis acid sites.
3
.3. Catalytic activity for glycerol esterification with acetic acid on parent, hierarchical
mordenite and Zr-modified analogs
Catalytic performance of the parent and hierarchical mordenites and their Zr-modified analogs
in esterification of glycerol with acetic acid is presented in Table 2. Acid treatment of the
initial mordenite leads to higher catalytic activity in esterification of glycerol most probably
because of the larger pores of hierarchical zeolite (1.6 nm for М1 comparing to 0.9 nm for М)
and the same acidity after acid treatment (Table 1). Additional modification with zirconia
oxide phase results in higher catalytic activity for both catalysts. The presence of strong
Brönsted and Lewis acid sites could be the reason for better catalytic performance. Higher
catalytic activity to more valuable triacetylglycerol is registered for М1 and Zr/М1 (Table 2)
which are favored in the larger pores of hierarchical mordenite and the presence of strong
Brönsted and Lewis acid sites. The high catalytic stability of the Zr/M1 is registered in 3
reaction cycles (Table 2) and it could be assigned to the simultaneous presence of Brönsted
and Lewis acid sites and the free access of the reactant’s molecules to them in the pores of
hierarchical mordenite.
4
. CONCLUSION
Hierarhical mordenite was prepared by etching with HF and NH4F. Zr-modifications of initial
and hierarchical mordenites were prepared by incipient wetness impregnation. The obtained
catalysts show high catalytic activity in glycerol esterification with acetic acid and selectivity
to valuable triacetylglycerol. Investigation of the acidic properties of the Zr-modified catalysts
revealed that the presence of sufficient amounts of Brönsted and Lewis acid sites with strong
acidity is of crucial importance for their catalytic performance. Zr-modified hierarhical
mordenite shows the best catalytic performance in the studied reaction, most probably due to
the optimal Brönsted/Lewis acid sites ratio and better access to them. The Zr-modified
hierarchical mordenite is a highly active, cost effective, stable and reusable catalyst.

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Acknowledgements Financial support from the COST action TD 1203 and by the NSF of
Bulgaria (Contract ДКОСТ 01/5). The authors Yu. Kalvachev and T. Todorova acknowledge
the financial support from ‘‘Program for career development of young scientists, Bulgarian
Academy of Sciences’2016’’ (Contract No. DFNP 182). The authors Venkata D. B. C.
Dasireddy, Blaž Likozar acknowledge the financial support from the Slovenian Research
Agency (research core funding No. P2-0152).
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Captions of the figures
Figure 1. XRD patterns of the studied samples
Figure 2. SEM images of parent mordenite (А) and the obtained by acid treatment hieratical
mordenite (В)
2
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1
29
Figure 3. Si MAS (blue line) and H- Si CPMAS (red line) NMR spectra of the studied
samples
Figure 4. FT-IR spectra of the adsorbed pyridine on the studied samples

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Table 1. Textural and acidity properties of the parent mordenite, hierarchical mordenite and
their Zr-modified analogs.
Samples
M
M1
372
0.30
1.6
Zr/M
302
0.16
0.9
Zr/M1
289
2
BET, m /g
3
0
0
6
72
3
Pore volume, cm /g
.16
.9
0.18
1.2
Pore size, nm
o
Amount of NH3
adsorbed
Up to 175 C,
.2
7.4
6.4
7.3
mmol/g
o
(mmol/g)
175-510 C,
8
3
.9
.5
8.4
3.7
7.2
3.4
10.5
4.0
mmol/g
o
5
10-740 C,
mmol/g
Total, mmol/g
1
1
8.6
.7
19.5
1.9
17.0
0.5
21.8
0.7
Brönsted/Lewis ratio, calculated
from FT-IR spectra
Table 2. Catalytic properties of the parent mordenite, hierarchical mordenite and their Zr-
o
modified analogs at 100 C and 3 hours reaction time.
Samples
Glycerol
Selectivity to
MAG,
%
Selectivity to Selectivity to
conversion, %
DAG,
%
TAG,
%
M
M1
68.2
89.3
74.4
93.5
65.1
90.6
13.1
2.6
52.5
63.7
53.2
69.2
43.6
66.1
19.3
7.0
Zr/M
17.6
3.6
Zr/M1
18.8
5.4
Spent Zr/M*
Spent Zr/M1*
25.5
30.9
12.4
21.5

Activity and selectivity in 3 reaction cycles

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*
*
*
*
*
zeolite mordenite
*
*
*
*
*
*
*
*
*
**
*
*
M1
Zr/M1
M
Zr/M
1
0
20
30
40
50
o
2
Theta ( )
Figure 1
Figure 2

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Zr/M
Zr/M1
M1
M
Figure 3

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B
B
543
1640
L
1
L
1610
1448
M1
M
Zr/M
Zr/M1
1
650
1600
1550
1500
1450
-
1
Wavenumbers (cm )
Fig. 4

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Graphical abstract

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Research Highlights




Hierarchical mordenite was prepared by etching with HF and NH4F
Parent and hierarhical mordenite were modified with zirconia by incipient wetness impregnation
The obtained materials were active in esterification of glycerol with acetic acid
Zr-hierarchical mordenite is a promising catalyst for glycerol valorization