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ORIGINAL ARTICLE
Effect of Fe-Mo promoters on HZSM-5 zeolite
catalyst for 1-hexene aromatization
Andrii Kostyniuk
a,*
, David Key
b
, Masikana Mdleleni
b
a
Department of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, Ljubljana
1001, Slovenia
b
PetroSA Synthetic Fuels Innovation Centre, South African Institute for Advanced Materials Chemistry, University of the Western
Cape, Cape Town, Private Bag X17, Bellville, 7535, South Africa
Received 23 May 2018; revised 9 October 2018; accepted 7 November 2018
Available online 15 November 2018
KEYWORDS
HZSM-5;
Fe-Mo species;
Olefins;
Aromatization;
1-Hexene;
Liquid Fuels
Abstract The promotional effect of Fe-Mo species introduced into HZSM-5 (Zeolyst Int.,
SiO
2
/Al
2
O
3
30) zeolite catalyst by the wetness impregnation method for the 1-hexene aromatiza-
tion was investigated. The structure and catalytic performance for the aromatization of 1-hexene
over xFeyMo-ZSM-5 catalysts in comparison with unmodified HZSM-5 catalysts were studied.
The xFeyMo-ZSM-5 catalysts contain fixed loading (5 wt%) and variable Fe/Mo ratio. The cata-
lysts were characterized by BET, ICP-AES, HRSEM-EDS, HRTEM, XRD, FTIR, H
2
-TPR,
NH
3
-TPD, and pyridine DRIFT spectroscopy. The characterization data confirmed the existence
of Fe and Mo species in the zeolite matrix. With Fe and Mo species implementation to HZSM-5
zeolite, the amount of the acid sites decreased, but the selectivities to C
9+
aromatics increased.
The catalyst evaluation was performed at 350 °C for 6 h on-stream at atmospheric pressure using
a fixed-bed quartz tube reactor. The selectivity to products of different carbon number was affected
by the Fe/Mo ratio within the zeolite. It was found the product distribution of grouped fractions of
C
1
–C
17+
from the liquid product. The results indicate that the optimum ratio of Fe/Mo is 1–1.5.
The highest selectivity for gasoline and distillate ranges was obtained for the 2.5wt%Fe2.5wt%
Mo- and 3wt%Fe2wt%Mo-ZSM-5 samples, which was higher than that for parent HZSM-5 cata-
lyst.
Ó2018 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under
the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
An important industrial way for the olefins upgrading is the
aromatization and skeletal hydroisomerization of olefins over
acid catalyst [1–6]. It is well-known that the aromatization of
olefins has great significance in the chemical industrial fields
for synthetic resin, rubber, solvent, detergent, clean gasoline
and other chemical intermediates [5]. The most investigated
olefins are propene and butene using phosphoric impregnated
*Corresponding author.
E-mail addresses: andry_kost@ukr.net (A. Kostyniuk), dkey@uwc.ac.
za (D. Key), masikana.mdleleni@petrosa.co.za (M. Mdleleni).
Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
Journal of Saudi Chemical Society (2019) 23, 612–626
King Saud University
Journal of Saudi Chemical Society
www.ksu.edu.sa
www.sciencedirect.com
https://doi.org/10.1016/j.jscs.2018.11.001
1319-6103 Ó2018 King Saud University. Production and hosting by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
silica–clay acid and zeolite type of catalysts [7–11], but studies
dealing with 1-hexene transformation are consequently limited
[12]. The products of this process are mixtures of olefins,
straight chain paraffins, cycloalkanes, and aromatics which
are contained in gasoline (C
5
–C
10
) and diesel (C
10
–C
20
)
[10,11]. 1-hexene aromatization can undergo many reactions
over the zeolites, such as cracking, oligomerization, isomeriza-
tion, cyclization, as well as hydrogen transfer over the
Brønsted and Lewis acid sites of the zeolite [5,6]. In general,
low temperatures (250 °C) favored the double bond shift reac-
tion but for the cracking is the major reaction at temperatures
above 450 °C[6]. The balanced aromatization and hydroiso-
merization reactions at moderate temperatures among 300–
400 °C appear to be attractive for achieving the desired
RON (research octane number) and gasoline yield simultane-
ously in the olefins upgrading [1].
Zeolite catalysts are widely uses for aromatization of olefins
[8,10,13]. ZSM-5 zeolite is one of the best solid acid catalysts
and has been widely applied in petrochemical industry, due
to its unique structures, thermal stability, acidity and shape
selectivity [1,2,14]. Many researchers studied the aromatization
process over ZSM-5 modified by Pt, Ga, and Zn, with the goal
of reducing coke formation, which leads to rapid catalyst deac-
tivation and poor stability [3–6]. In addition, the strength of
acid sites in promoted zeolite catalysts, which can catalyze
the aromatization of olefins, is still unclear. Nevertheless, the
results of these investigations suggest that the development
of a more active catalyst for aromatization of 1-hexene is
needed and still remains a major problem [3,15].
In this paper, the aim of this study was to prepare and
investigate Fe-Mo bimetal zeolite catalysts and study the effect
of adding metal species for the 1-hexene aromatization, which
has never been reported before. For such a purpose, Fe-Mo
species were incorporated into HZSM-5 zeolite by the wetness
impregnation method and can be used to enhance zeolite activ-
ity during the 1-hexene aromatization. The effects of Fe/Mo
ratio on the catalysts activity and product distribution will
be evaluated.
2. Experimental
2.1. Chemicals used
Reagents that have been used in the preparation of the catalyst
and the catalytic reaction: NH
4
-ZSM-5 zeolite (Zeolyst Int.,
CBV 3024E), iron(III) nitrate nonahydrate (Sigma-Aldrich,
98%), ammonium heptamolybdate tetrahydrate (Sigma-
Aldrich, 99.98%), 1-hexene (Sigma-Aldrich, 97%).
2.2. Catalyst synthesis
The preparation of the xFeyMo-ZSM-5 catalysts has involved
the impregnation of the commercial NH
4
-ZSM-5 zeolite (SiO
2
/
Al
2
O
3
30) using the incipient wetness impregnation method
with solutions of ammonium heptamolybdate (NH
4
)
6
Mo
7
O
24
-
4H
2
O and iron(III) nitrate Fe(NO
3
)
3
9H
2
O. At first, the
NH
4
-ZSM-5 zeolite was calcined at 550 °C in static air for
6 h to convert the ammonium form to its protonated form
(H-ZSM-5). Samples containing both molybdenum and iron
were prepared by a two-step impregnation procedure, in which
the molybdenum phase with a concentration of 0.001 M was
introduced first to the H-ZSM-5 and mixed for 2 h at 80 °C.
After that, the iron salt with a concentration of 0.01 M was
added to the prepared solution with vigorous stirring. The
temperature was kept at 80 °C. Stirring continued until suspen-
sion turned into a slurry. Drying was carried out at 120 °Cin
the ventilated dryer overnight. Thereafter calcination in the
static air was carried out for 6 h at 550 °C. 10 different cata-
lysts based on H-ZSM-5 were prepared in this way with the
following amounts of metals: 5wt%Mo, 1wt%Fe4wt%Mo,
1.25wt%Fe3.75wt%Mo, 1.43wt%Fe3.57wt%Mo, 1.67wt%
Fe3.33wt%Mo, 2wt%Fe3wt%Mo, 2.5wt%Fe2.5wt%Mo,
3wt%Fe2wt%Mo, 4wt%Fe1wt%Mo and 5wt%Mo. All per-
centages associated with metal amount are expressed as weight
percentage.
2.3. Catalyst characterization
2.3.1. Fourier transform infrared (FT-IR) spectroscopy analysis
The FTIR framework spectra were obtained using Perkin
Elmer UATR FTIR spectrometer. Typically 20 mg of sample
was placed on the ATR diamond crystal and the force gauge
set to 50 (arb. units) to obtain good contact between sample
and crystal. The samples were recorded in the region 400 to
4000 cm
1
.
2.3.2. Surface area and micropore analysis (BET)
Brunauer–Emmett–Teller (BET) specific surface area was
determined using a Micrometrics 3300, TriStar surface area
and porosity analyzer. About 0.3 g of sample was degassed
at 400 °C for 4 h. After the degassing process, the samples were
then loaded on the analysis station for determination of the
isotherms at 196 °C. The pore size distributions were calcu-
lated using the Barrett–Joyner–Halenda (BJH) model applied
to the adsorption branch of the isotherm, assuming cylindrical
pore geometry.
2.3.3. ICP-AES analysis
The chemical analysis of elements’ (Fe and Mo) amount in cat-
alysts was determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) on Varian model 715-ES.
Prior to the ICP-AES measurements, the zeolite samples were
dissolved in a mixture of HF/HNO
3
/H
2
O (1:1:1).
2.3.4. HRSEM-EDS analysis
Scanning electron microscopy (SEM) micrographs were
obtained using a high-resolution SEM EHT 5.00 kV. All sam-
ples were carbon coated before imaging. The HRSEM (AUR-
IGA) was also equipped with an EDS spectrometer with the
INCA EDS system by Oxford Instruments for elemental anal-
ysis of zeolites.
2.3.5. HRTEM analysis
Transmission electron microscopy images were obtained using
the aid of HRTEM techniques using a FEI Tecnai TF20
(200 kV) equipped with a STEM unit, high-angle annular
dark-field (HAADF) detector and X-Twin lenses.
2.3.6. Powder X-ray diffraction analysis
Powder X-ray diffraction data were collected using a Brucker
AXS D8 diffractometer equipped with a primary beam Gobel
Effect of Fe-Mo promoters on HZSM-5 zeolite catalyst 613
mirror, a radial Soller slit, a Vantec-1 detector and using
Cu-Karadiation (40 kV, 40 mA, kKa1 = 1.5406 A
˚). Data
were collected in the 2hrange 5 to 70°and 90°in 0.021°steps,
using a scan speed resulting in an equivalent counting time of
14.7 s/step.
2.3.7. H
2
-TPR analysis
Temperature programmed reduction (TPR) characteristics of
the catalysts were obtained using a Micrometrics Autochem
2920 II system. Sample (0.1 g) was loaded in a U-shaped
quartz tubular reactor. When performing the TPR analysis,
the sample was exposed to a gas mixture (10% H
2
, balance
Ar) at a flow rate of 50 mL/min and heated to 900 °Cat
10 °C/min heating rate. The H
2
consumption was monitored
with a thermal conductivity detector (TCD). A cooling trap
(ice + isopropanol) placed between the sample and the TCD
was used to retain the water produced during the reduction
process.
2.3.8. NH
3
-TPD analysis
The acid properties of the catalysts were determined by
temperature-programmed desorption of ammonia (NH
3
-
TPD) on a Micrometrics Autochem 2920 II system using He
as a carrier gas. All samples (0.1 g) were first heated to
500 °C and then cooled down to 120 °C in He atmosphere.
Afterward, 5 vol% NH
3
in He (20 mL/min) was adsorbed at
120 °C for 30 min followed by He purging at the same temper-
ature for 1 h. Desorption of NH
3
was monitored in the range
of 120–700 °Cata10°C/min heating rate. The NH
3
desorp-
tion profile was observed using a thermal conductivity
detector.
2.3.9. Pyridine-DRIFT analysis
Thermogravimetric method of pyridine adsorption was used
for quantification of acid sites on investigated materials using
Pyris 1 TGA apparatus from PerkinElmer. Prior to analysis,
the samples were degassed in-situ in N
2
stream (30 mL/min)
at 500 °C for 1 h. Afterward, the sample was cooled to
125 °C and saturated with pyridine vapors by passing the N
2
stream through a saturator filled with liquid pyridine. Satura-
tion was followed by desorption of weakly bound pyridine by
degassing the sample in N
2
at 125 °C for an additional 2 h until
achieving a stable sample weight. The total number of acid
sites was calculated based on the weight difference before
and after sample saturation.
Differentiation between Brønsted (BAS) and Lewis (LAS)
acid sites was done for pure HZSM-5 and 2.5Fe2.5Mo-
HZSM-5 materials by DRIFTS analysis using Frontier IR
spectrometer (Perkin Elmer), DiffusIRÒaccessory from Pike
Scientific and pyridine as the probe molecule. The powdered
samples (10 mg) were positioned in the sample cup and pre-
treated in N
2
(50 mL/min) at 500 °C for 30 min. After cool-
ing to 125 °C, the samples were saturated with pyridine
vapors for 10 min, followed by degassing in a vacuum
(2 10
5
mbar) for 30 min. Spectra were recorded with 8
accumulations and spectral resolution of 4 nm between 800
and 4000 cm
1
. For characterization of LAS and BAS,
absorption bands at 1445 cm
1
and 1545 cm
1
were
considered.
2.4. Catalytic evaluation
The performances of the zeolite catalysts were tested in the
conversion of 1-hexene in a fixed bed quartz tube reactor at
a weight hourly space velocity (WHSV) of 4 h
1
and the reac-
tion temperature was kept at 350 °C under atmospheric pres-
sure. The reactor tube was charged with 1 g of a catalyst
then heated from room temperature up to 350 °C. 1-hexene
was introduced by New era syringe pump at a flow rate of
0.098 mL/min with the syringe of 29 mm diameter continu-
ously for 6 h. All products were analyzed on an offline Bruker
450 GC equipped with a BR-Alumina/Na
2
SO
4
column (C
1
–
C
6
), BR-1 column (C
7
–C
17
) and a flame ionization (FID)
detector. 1-hexene conversion and selectivity were defined as:
X1hexeneðmol%Þ
¼ð1-hexene moles in the feedÞ-ð1-hexene moles in the productsÞ
ð1-hexene moles in the feedÞ
SCxHyðmol%Þ¼ CxHy moles in the products
PCxHy moles in the products 100
3. Results and discussion
3.1. Catalyst characterization
3.1.1. Surface area and micropore analysis (BET)
Table 1 summarizes the nominal composition of samples with
different Mo/Fe ratio and fixed loading (5wt%). The BET sur-
face area (S
BET
), microporous surface area (S
micro
), external
surface area (S
extrer
), microporous volume (V
micro
), total pore
volume (V
total
), average pore diameters and hierarchy factor
(HF) of the studied catalysts are listed in Table 2 [16].
Correspondingly, the micropore volume of promoted
HZSM-5 zeolite was reduced from 0.105 to 0.094 cm
3
/g, as
revealed in Table 2. After loading 5wt% of Mo on HZSM-5,
the S
BET
decreased from 378 to 314 cm
2
/g due to strong Mo
interaction with HZSM-5 and indicating that Mo mostly
deposited on the external surfaces and could have blocked
some of the micropores of HZSM-5 [16–18]. Upon addition
Table 1 Nominal composition for prepared samples of
xFeyMo-ZSM-5 catalysts.
Sample xFe
(wt%)
yMo
(wt %)
Mo/Fe
ratio
Total load
(wt%)
Fixed loading and variable Mo/Fe ratio
5Mo 0 5 15
1Fe4Mo 1 4 4 5
1.25Fe3.75Mo 1.25 3.75 3 5
1.43Fe3.57Mo 1.43 3.57 2.5 5
1.67Fe3.33Mo 1.67 3.33 2 5
2Fe3Mo 2 3 3/2 5
2.5Fe2.5Mo 2.5 2.5 1 5
3Fe2Mo 3 2 2/3 5
4Fe1Mo 4 1 1/4 5
5Fe 5 0 0 5
614 A. Kostyniuk et al.
of 5wt% of Fe the S
BET
decreased to 346, but not so dramat-
ically comparing with 5wt%Mo.
The xFeyMo-ZSM-5 samples noticed a different behavior
with variable Mo/Fe ratio. The xFeyMo-ZSM-5 samples exhi-
bit a greater surface area than 5Mo-ZSM-5 suggesting a better
dispersion when Fe and Mo metals are both present [16,18].
Fig. 1(a, b, c) provides the N
2
adsorption-desorption iso-
therms for samples with different Mo/Fe ratio. The character-
istic of the xFeyMo-ZSM-5 samples tending to agglomerate
into microsized agglomerates is also featured by its N
2
adsorp-
tion and desorption isotherms, which could also be supported
by the XRD and SEM results [17]. As shown in Fig. 1 all cat-
alysts exhibit typical type I isotherms with a small hysteresis
loop in the range of p/p
0
= 0.5–0.9, which indicate that the
HZSM-5 and xFeyMo-ZSM-5 samples have high microporos-
ity and inter-crystal mesoporosity possibly related to the
aggregation of the crystals [3,17,19–22].
Compared with the isotherm of the HZSM-5, for the
xFeyMo-ZSM-5 samples, the N
2
adsorption decreased. A little
difference in the shape of N
2
adsorption/desorption isotherms
for xFeyMo-ZSM-5 samples was observed, but the presence of
both Fe and Mo on the external surfaces of zeolite crystals has
little influence on their agglomeration behaviors [17].
The hierarchy factor (HF) can be applied to any material,
to classify the porous characteristic [23,24]. The hierarchy fac-
tors were calculated according to Eq. (1).
HF ¼Sexter=SBET
ðÞVmicro=Vtotal
ðÞ ð1Þ
Well-known conventional zeolites display moderate HF
value (0.1) and hierarchical zeolites display relatively high
HF value (>0.1) due to the introduction of mesopores. The
HF value of the metals promoted and parent ZSM-5 is
0.158–0.178 (>0.1), that is, displaying high relative meso-
porosity, in good agreement with the TEM images [23].
The pore size distribution was obtained by applying the
Barrett–Joyner–Halenda (BJH) method from the adsorption
branches of nitrogen isotherms [25].Fig. 1d shows the meso-
pores with sizes in a range of 12.5–21.0 nm [3]. The majority
of these pores concentrated at about 14 nm. Compared with
HZSM-5 catalyst, there was a notable increase in the pore size
distribution for the 2.5Fe2.5Mo-ZSM-5 (14.5 nm), 3Fe2Mo-
ZSM-5 (14.5 nm), 4Fe1Mo-ZSM-5 (12.5 nm) and 5Fe-ZSM-5
(21.0 nm) samples, exhibiting that the increase in incorpora-
tion of Fe-Mo species in the framework made the pore
diameter larger. Thus, impregnation does not change the
Table 2 Characterization data of the zeolite samples.
Catalyst Fe
a
(wt%)
Mo
a
(wt%)
S
BETb
(m
2
/g)
S
microb
(m
2
/g)
S
exterb
(m
2
/g)
V
microc
(cm
3
/g)
V
totalc
(cm
3
/g)
BJH Adsorption
average pore
diameter
d
(nm)
BJH Desorption
average pore
diameter
d
(nm)
HF
HZSM-5 – – 378 228 150 0.105 0.235 5.92 5.75 0.178
5Mo – 4.74 314 207 107 0.096 0.208 6.13 5.81 0.158
1Fe4Mo 0.92 3.82 326 203 123 0.094 0.215 6.24 5.95 0.165
1.25Fe3.75Mo 1.13 3.65 330 205 125 0.095 0.218 5.73 5.50 0.165
1.43Fe3.57Mo 1.40 3.46 324 204 120 0.095 0.211 5.56 5.31 0.167
1.67Fe3.33Mo 1.61 3.10 339 207 132 0.096 0.223 5.67 5.48 0.168
2Fe3Mo 1.94 2.85 316 203 113 0.094 0.210 6.83 4.41 0.161
2.5Fe2.5Mo 2.47 2.36 331 204 127 0.094 0.221 6.01 5.75 0.164
3Fe2Mo 2.91 1.89 339 217 122 0.100 0.222 5.82 5.60 0.162
4Fe1Mo 3.88 0.89 348 212 136 0.098 0.230 5.82 5.62 0.167
5Fe 4.78 – 346 209 137 0.097 0.228 5.89 5.68 0.168
a
ICP-AES.
b
BET method.
c
t-Plot method.
d
BJH method.
Fig. 1 N
2
-adsorption and desorption isotherms (a, b, c) and BJH pore size distribution (d) of the H-ZSM-5 and xFeyMo-ZSM-5
samples.
Effect of Fe-Mo promoters on HZSM-5 zeolite catalyst 615
texture of catalysts which agrees with XRD, FTIR, SEM and
TEM results [26].
3.1.2. Electron microscopy and X-ray microanalysis
The HRSEM-EDS images of the HZSM-5 and xFeyMo-ZSM-
5 zeolite catalysts are shown in Fig. 2.
The SEM images showed aggregates of particles. No major
morphological differences were observed between zeolite cata-
lysts. Fig. 2 shows two typical SEM images obtained. It is clear
from the images that the zeolite catalysts have a cube or elon-
gated prismatic shapes [16,17,19,20].
The element mapping of the HZSM-5, 5Fe-ZSM-5, 5Mo-
ZSM-5 and 2.5Fe2.5Mo-ZSM-5 samples by HRSEM is shown
in Fig. 3.
From Fig. 3 we can see that the distribution of the element
Fe and Mo the same as Si and Al is homogeneous. The element
analysis shows that the O–K, Al–K, Si–K, Fe–K, Fe–L, Mo–K
and Mo–L signals display the elements are uniformly dis-
tributed. In other words, the iron and molybdenum species
Fig. 1 (continued)
616 A. Kostyniuk et al.
are well dispersed on the external surface of the HZSM-5 crys-
tals [27].
3.1.3. The high-resolution transmission electron microscopy
(HRTEM)
HRTEM and Energy Dispersive Spectroscopy (EDS) were
performed for the HZSM-5 and xFeyMo-ZSM-5 samples to
confirm the presence of iron and molybdenum species in
HZSM-5 and study the composition of the metallic phase in
the catalysts. Fig. 4 shows brightfield TEM images. The pres-
ence of Fe and Mo particles in these objects is also evidenced
by EDS analysis, as it is shown in Fig. 5, where the peaks cor-
responding to O–K, Al–K, Si–K, Fe–K, Fe–L, Mo–K and
Mo–L emissions are clearly outlined together with other peaks
that arise from the carbon and the copper grid [28,29].Asitis
seen from EDS spot area the Fe and Mo were always detected
together and involved in the formation of iron and molybde-
num species in the HZSM-5 zeolite catalyst [17].
3.1.4. Fourier transform infrared (FT-IR) measurements
The FT-IR spectra of the HZSM-5 and xFeyMo-ZSM-5 sam-
ples were recorded in the range of 4000–400 cm
1
and shown
in Fig. 6. The absorption bands at 1220, 1075, 797, 542,
433 cm
1
are considered as the characteristic signals for the
framework vibration of the HZSM-5 zeolite catalyst [30].It
was found that the band 433 cm
1
belongs to the T–O bending
vibration of internal tetrahedral (where T = Si or Al),
542 cm
1
(double ring), 797 cm
1
(external symmetric stretch),
1075 cm
1
(internal asymmetric stretch) and 1220 cm
1
(exter-
nal asymmetric stretch), respectively [26,30–32].
According to the IR spectra recorded from the xFeyMo-
ZSM-5 samples, all the structure-sensitive bands are similar
Fig. 2 SEM images of the HZSM-5 and xFeyMo-ZSM-5 samples with fixed loading and variable Mo/Fe ratio (A – HZSM-5, B – 5Mo,
C – 1Fe4Mo, D – 1.25Fe3.75Mo, E – 1.43Fe3.57Mo, F – 1.67Fe3.33Mo, G – 2Fe3Mo, H – 2.5Fe2.5Mo, I – 3Fe2Mo, J – 4Fe1Mo, K – 5Fe).
Effect of Fe-Mo promoters on HZSM-5 zeolite catalyst 617
to those of HZSM-5, but for the 5Mo-ZSM-5 catalyst, a new
band was found.
The spectrum of xFeyMo-ZSM-5 catalysts shows major
changes in the region of 750–1000 cm
1
, weakening of strong
absorption of the band at 797 cm
1
. A new broad band was
found at 901 cm
1
, which corresponds to the overlap between
Mo-O-Mo bond vibrations in MoO
3
. The band at 1075 cm
1
is sensitive to the ratio of framework Si/Al. By the loading
of Fe and Mo species, the band at 1075 cm
1
shifted to
1081 cm
1
(Fig. 6). Thus, the FTIR spectra of HZSM-5 were
not significantly affected after loading the Fe and Mo species,
which means the introduction of these species did not change
the HZSM-5 basic framework [33].
3.1.5. X-ray diffraction (XRD) analysis
Figs. 7-8 shows the XRD patterns of samples with different
Fe/Mo ratios. The similarity between the XRD patterns of
the HZSM-5 and xFeyMo-ZSM-5 samples indicates that the
HZSM-5 framework was preserved after impregnation
[16,27,34,35]. In contrast, the samples impregnated with pure
Fe, Mo, and Fe-Mo species do not exhibit any reflections
related to the iron and molybdenum species indicating very
small particles size [30]. As shown in Figs. 7, 8 the Fe-Mo addi-
tives cause a decrease in the HZSM-5 crystallinity since a
reduction of the HZSM-5 characteristic peaks are observed.
Once all the samples compared with HZSM-5 exhibited a
decrease in the peaks between 2h= 7.6°–9.2°and 2h=22°–
29°, it is reasonable to suppose that the dispersion of iron
and molybdenum species takes place on the HZSM-5 surface
as well as inside the channels.
3.1.6. H
2
–TPR analysis
H
2
–TPR experiments were conducted to investigate the
reducibility of the metals modified H-ZSM-5 catalysts. In
Fig. 2 (continued)
618 A. Kostyniuk et al.
Fig. 9, the temperature programmed reduction profiles for Fe,
Mo and Fe-Mo loaded catalysts are shown. The catalysts exhi-
bit different temperature-programmed reduction profiles indi-
cating the different interaction between active phases [36].
H
2
-TPR of unmodified H-ZSM-5 did not show hydrogen con-
sumption in the investigated temperature range [37]. The main
reduction peaks corresponding to the metal phase reduction
were located between 305 °C and 731 °C. The addition of
Mo species in the supported Fe oxide catalyst system shifted
the H
2
-TPR peaks to higher reduction temperatures, showing the
aH
2
-TPR peak at 426 °C for the 2.5Fe2.5Mo-ZSM-5 sample [38].
In Fig. 9, three reduction peaks arising from the
2.5Fe2.5Mo-ZSM-5 catalyst can be distinct, the one at
426 °C shows a reduction of Fe
2
O
3
to Fe
3
O
4
, and the others
at 523 and 731 °C reduction of Fe
3
O
4
to FeO and Fe and inter-
action with Mo specious [37,39,40].
For the 5Mo-ZSM-5, the H
2
consumption curve deviates
from the baseline at approximately 300 °C, and the H
2
-TPR
peak occurred at above 429, 485 and 668 °C[38]. The peak
at 429 and 485 °C is attributed to the reduction of the species
Mo
+6
to Mo
+4
, from the MoO
3
polymeric species. The peak
at 668 °C is assigned to the reduction of the Mo
2+
phase to
Mo
0
[28]. The H
2
-TPR curve for the 5Fe-ZSM-5 sample also
showing three peaks at 305, 475 and 598 °C with involving
the reduction of the iron oxide in a three-step reduction pro-
cess: Fe
2
O
3
?Fe
3
O
4
?FeO ?Fe, respectively.
3.1.7. NH
3
-TPD analysis
The concentration and strength of acid sites in HZSM-5 and
2.5Fe2.5Mo-ZSM-5 are determined by NH
3
-TPD, as pre-
sented in Fig. 10 and the quantitative result is listed in Table 3.
Fig. 3 Element mapping of the HZSM-5 (A), 5Fe-ZSM-5 (B), 5Mo-ZSM-5 (C), 2.5Fe2.5Mo-ZSM-5 (D).
Effect of Fe-Mo promoters on HZSM-5 zeolite catalyst 619
Two desorption peaks were observed for the parent HZSM-5
at 191 and 389 °C. The peak at 191 °C has been assigned to
desorption of the absorbed NH
3
on a weak acid (mostly
Lewis-acid) sites, whereas, the peak at 389 °C was assigned
to medium strong acid (mostly Bronsted-acid) sites
[16,27,41]. With the Fe and Mo species were implemented,
the area of weak and strong acidic sites decreased [42]. The
strong acid sites change more than the weak ones. At the same
time, the 2.5Fe2.5Mo-ZSM-5 sample showed NH
3
-TPD peaks
with lower desorption temperatures in comparison with that of
the parent HZSM-5 zeolite. The 2.5Fe2.5Mo-ZSM-5 exhibits
obviously weaker acidity than the HZSM-5 catalyst.
The decrease in the amount of strong acid sites results in a
decrease in the selectivities to benzene and toluene and an
increase in the selectivity to C
9+
aromatics [42]. The surface
acid sites, especially strong acid sites, facilitated the second
Fig. 4 HRTEM images of the xFeyMo-ZSM-5 catalysts with fixed loading and variable Mo/Fe ratio (A – HZSM-5, B – 5Mo, C –
1Fe4Mo, D – 1.25Fe3.75Mo, E – 1.43Fe3.57Mo, F – 1.67Fe3.33Mo, G – 2Fe3Mo, H – 2.5Fe2.5Mo, I – 3Fe2Mo, J – 4Fe1Mo, K – 5Fe).
620 A. Kostyniuk et al.
reaction of products and coke formation. Therefore, the
distinct surface acid strength and acid site density may lead
to different catalytic activities of the 2.5Fe2.5Mo-ZSM-5 cata-
lyst for the 1-hexene aromatization reaction [38,43].
3.1.8. Pyridine-DRIFT analysis
For characterization of LAS and BAS, absorption bands at
1445 cm
1
and 1545 cm
1
were considered. The first band
originates from pyridine coordinative adsorbed on LAS, while
the second arises from pyridinium ion bound to BAS. The
BAS/LAS ratio was calculated as:
CBAS=CLAS ¼1:73=1:23 IBAS =ILAS
In this equation, I
BAS
and I
LAS
represent the intensity of
absorption bands at 1545 and 1445 cm
1
, and 1.73 and 1.23
are relevant extinction coefficients, as reported by Tamura
et al. [44]. When Fe-Mo species were deposited over HZSM-
5, the amount of acid sites decreases slightly, from 0.63 to
0.58 mmol/g
cat
. This could be connected either to pore block-
age and inaccessibility of acid sites to the pyridine probe mole-
cule what was verified by N
2
physisorption. On a side note, the
total amount of acid sites assuming all framework Al
3+
is
1.08 mmol/g. Notably less was measured, suggesting inaccessi-
bility of all acid sites for the probe molecule.
Pyridine DRIFT analysis showed that pure HZSM-5 con-
tains mainly BAS (BAS/LAS = 4). A notable fraction (20%)
of aluminum appears as LAS, which suggests their extra-
framework location. After Fe-Mo species deposition, a notable
increase in LAS is observed (BAS/LAS = 2.7, Table 4). Based
on this, the addition of Mo species enhanced the surface weak
acid strength in the 2.5Fe2.5Mo-ZSM-5 catalyst [38]. This is
anticipated since coordinatively unsaturated Mo
6+
in MoO
x
clusters can exhibit Lewis acidic character.
3.2. Catalyst evaluation
The effect of Fe/Mo ratio on the catalytic conversion of 1-
hexene was studied at a reaction temperature of 350 °C for
6 h on-stream. The reaction was performed at atmospheric
pressure. The conversion of 1-hexene was observed 99% over
Fig. 5 A typical Energy Dispersive Spectrum (EDS) of the
2.5Fe2.5Mo-ZSM-5 sample.
Fig. 6 FT-IR spectra of the HZSM-5 and xFeyMo-ZSM-5 catalysts.
Effect of Fe-Mo promoters on HZSM-5 zeolite catalyst 621
Fig. 7 XRD patterns of the HZSM-5 and 5Fe-, 5Mo-, 1Fe4Mo- and 2.5Fe2.5Mo-ZSM-5 catalysts.
Fig. 8 XRD patterns of the HZSM-5, 1.25Fe3.75Mo-, 1.67Fe3.33Mo-, 2Fe3Mo-, 3Fe2Mo- and 4Fe1Mo-ZSM-5 catalysts.
622 A. Kostyniuk et al.
xFeyMo-ZSM-5 and HZSM-5 catalysts, showing that 1-
hexene is a very active feed.
Fig. 11 shows that a high gasoline selectivity (>40%) was
observed for the 2.5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5
catalysts.
The parent HZSM-5 with other promoted catalysts being
kept at 40% and less. The best selectivity for C
6
-C
9
was
obtained over 3Fe2Mo-ZSM-5 catalyst, but C
10+
(>25%)
over 2.5Fe2.5Mo-ZSM-5 and being higher (9wt%) than parent
HZSM-5 catalyst after TOS = 6 h.
Fig. 12 shows the product distribution of grouped fractions
of C
1
-C
17+
from the liquid product for the HZSM-5,
2.5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5 catalysts.
The product distribution of C
8
,C
9
and C
10
was 15.1%,
20.5% and 11.9% respectively for the HZSM-5 catalyst. Upon
addition of Fe-Mo to the parent H-ZSM-5 catalyst the product
distribution of C
8
,C
9
and C
10
increased to 11.3%, 31.2%,
23.0% for the 2.5Fe2.5Mo-ZSM-5 and 18.9%, 30.4%,
18.2% for the 3Fe2Mo-ZSM-5, respectively.
The strong interaction between the Fe and Mo species in
the 2.5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5 catalysts was
estimated, which resulted in a high chemical value of
Mo
6+
formed in Fe–Mo bimetal oxide system [38]. Moreover,
the catalytic performance of xFeyMo-ZSM-5 catalysts is
affected by Fe/Mo molar ratio. This molar ratio has a
significant influence on the surface acid site density. The main
reason for the enhanced activity of the 2.5Fe2.5Mo-ZSM-5
and 3Fe2Mo-ZSM-5 catalysts is related to the modification
Fig. 10 NH
3
-TPD profiles of the HZSM-5 and 2.5Fe2.5Mo-
ZSM-5 catalysts.
Table 3 NH
3
-TPD results for the HZSM-5 and 2.5Fe2.5Mo-ZSM-5.
Catalyst Desorption Temperature (
o
C) Acidity by strength (mmol/g) Strong/weak
T
1
T
2
Weak Strong
H-ZSM-5 191 389 0.481 0.218 0.453
2.5Fe2.5Mo-ZSM-5 171 340 0.468 0.038 0.081
Fig. 11 Effect of Fe/Mo ratio with TOS = 6 h on the selectivity
of C
1
-C
5
,C
6
-C
9
,C
10+
hydrocarbons.
Fig. 9 H
2
-TPR profiles of the 5Fe-ZSM-5, 5Mo-ZSM-5 and
2.5Fe2.5Mo-ZSM-5 catalysts.
Table 4 Total acidity expressed as mmol pyridine chemi-
sorbed per gram of catalyst.
Sample Total acidity, mmol/g
cat
BAS/LAS
HZSM-5 0.63 4.0
2.5Fe2.5Mo-HZSM-5 0.58 2.7
Effect of Fe-Mo promoters on HZSM-5 zeolite catalyst 623
of the acidity H-ZSM-5 resulting in an increase in the Lewis
acid sites and a decrease in the BAS/LAS ratio, thus is
increased of the selectivity to C
9+
aromatics what is sup-
ported by pyridine-DRIFTS, NH
3
-TPD and catalyst evalua-
tion results [18].
The Fe species are important for incorporation to
Mo catalyst not only because of their promotional effects
and stability but because they are abundant, cheap and envi-
ronmentally friendly and can have industrial applications
[45]. We speculate that the electron transfer (strong interac-
tion) occurred between Fe and Mo species for 2.5Fe2.5Mo-
ZSM-5 and 3Fe2Mo-ZSM-5 catalysts and confirmed it by
H
2
-TPR. Unfortunately, until now no definitive conclusion
has been established regarding the effect of Fe additive on
Mo catalysts [45], but it can be concluded that the synergetic
effect between Fe and Mo species exists and to improve the
catalytic performance of 1-hexene aromatization reaction
[46].
4. Conclusions
The 1-hexene aromatization was studied on metal modified
HZSM-5 and compared with unmodified HZSM-5 (Zeolyst
Int., SiO
2
/Al
2
O
3
30). Modifying of the HZSM-5 catalyst
with Fe and Mo species could enhance the rate of aromatics
production in the 1-hexene aromatization reaction. At
350 °C, a variety of products were detected, where the C
8
and C
9
were the major products in the gasoline range and
C
10
was the major product in the distillate range. The
Fe-Mo modified H-ZSM-5 zeolites were prepared using the
incipient wetness impregnation method in a varying ratio. A
combination of catalytic testing and detailed catalyst charac-
terization resulted in the identification of effects on the activity
depending on the varying ratio of metals in the zeolite
catalysts. Adding of the Fe-Mo species to HZSM-5 has been
shown to be positive for the high selectivity of gasoline and dis-
tillate production both at standard experimental conditions
with using atmospheric pressure. The decrease in the amount
of strong acid sites results in an increase in the selectivity to
C
9+
aromatics. The 2.5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5
catalysts showed higher selectivity in gasoline and distillate
range than parent HZSM-5 and others promoted catalysts.
Thus, the synergetic effect of Fe and Mo species exists for
2.5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5 catalysts, which has
improved the catalytic performance of 1-hexene aromatization
reaction.
5. Declarations of interest
The authors declare no competing interests.
Acknowledgments
We are grateful to PetroSA for financial support. We are also
indebted to Dr. Petar Djinovic
ˇ(National Institute of Chem-
istry) for obtaining and description the pyridine-DRIFT spec-
tra of the H-ZSM-5 and 2.5Fe2.5Mo-ZSM-5 samples.
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