Seed-assisted OSDA-free synthesis of ZSM-5 zeolite and its application in dehydrogenation of propane

Article abstract of DOI:10.1016/j.materresbull.2015.01.062

ZSM-5 is one of the most well-known members of zeolite family owing to its huge number of industrial applications due to its instinct acidity and well-defined micropore structure. It can be synthesized in a one or multi-step hydrothermal procedure employing versatile synthesis parameters. Here, seed-assisted OSDA (organic structure directing agent)-free synthesis method based on using TPABr-silicalite-1 as structure directing agent is proposed as an economical route considering reproducibility and short synthesis time benefits as well. This two-step synthesis technique exploits an enriched aluminum template-free precursor gel containing zeolite nuclei that attach and grow around high siliceous zeolite cores named as silicalite-1. The prepared ZSM-5 product was characterized using XRD, SEM, FT-IR, EDX, elemental mapping, ICP and N₂ adsorption-desorption techniques. Results confirmed the fully crystalline pure MFI structure of the obtained product. The catalytic behavior of the fabricated material as support was evaluated in the propane dehydrogenation reaction (PDH). The PtSn-based ZSM-5 catalyst maintained its activity to a certain level of about 31% propane conversion after 7 h time on stream with more than 89% total olefins selectivity. It seems that seeding along with using a cheap template like TPABr in the synthesis of seeds is an efficient method which has the potential to be emerged as a promising method for the ZSM-5 large-scale production.

Full text of DOI:10.1016/j.materresbull.2015.01.062

Materials Research Bulletin 65 (2015) 253–259
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Seed-assisted OSDA-free synthesis of ZSM-5 zeolite and its application
in dehydrogenation of propane
*
Marjan Razavian, Shohreh Fatemi , Milad Komasi
School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box 11365-4563, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
ZSM-5 is one of the most well-known members of zeolite family owing to its huge number of industrial
applications due to its instinct acidity and well-defined micropore structure. It can be synthesized in a
one or multi-step hydrothermal procedure employing versatile synthesis parameters. Here, seed-
assisted OSDA (organic structure directing agent)-free synthesis method based on using TPABr-silicalite-
1 as structure directing agent is proposed as an economical route considering reproducibility and short
synthesis time benefits as well. This two-step synthesis technique exploits an enriched aluminum
template-free precursor gel containing zeolite nuclei that attach and grow around high siliceous zeolite
cores named as silicalite-1. The prepared ZSM-5 product was characterized using XRD, SEM, FT-IR, EDX,
elemental mapping, ICP and N2 adsorption–desorption techniques. Results confirmed the fully crystalline
pure MFI structure of the obtained product. The catalytic behavior of the fabricated material as support
was evaluated in the propane dehydrogenation reaction (PDH). The PtSn-based ZSM-5 catalyst
maintained its activity to a certain level of about 31% propane conversion after 7 h time on stream with
more than 89% total olefins selectivity. It seems that seeding along with using a cheap template like TPABr
in the synthesis of seeds is an efficient method which has the potential to be emerged as a promising
method for the ZSM-5 large-scale production.
Received 20 April 2014
Received in revised form 18 November 2014
Accepted 29 January 2015
Available online xxx
Keywords:
A. Microporous materials
B. Chemical synthesis
C. X-ray diffraction
D. Catalytic properties
ã 2015 Published by Elsevier Ltd.
1. Introduction
There are two general OSDA-free synthesis protocols regarding
ZSM-5 molecular sieve production namely template-free direct
It is more than six decades that lots of historical and hierarchical
improvements have been evolved in materials sieving industry
which were launched introducing aluminosilicate zeolites. Zeolites
are three-dimensional porous mineral or synthetic crystalline
materials with pore openings ranging from 0.2 to 30 nm which are
commonly used as adsorbents and catalysts [1,2]. ZSM-5 as the
most popular member of this family consist of SiO4 and AlO4
tetrahedra linking together forming pentasile chains that finally
construct MFI structure with two different types of channels with
dimensions around 5.5 Å. These pore openings besides its surface
acidity give it size and shape sieving and catalytic properties which
make it a great candidate for lots of chemical reactions like
cracking, dewaxing, MTO, etc. [3–5]. ZSM-5 was firstly introduced
by Argauer and Lendolt at mobile company in 1972 [6]. It can be
effectively synthesized from the hydrothermal crystallization of an
alkali aluminosilicate gel containing expensive template species as
organic structure directing agents or space fillers.
synthesis strategy in which an aluminosilicate gel is heated to form
ZSM-5 crystals [7] and seed-assisted template-free method in
which extraneous seeds are introduced to the aluminosilicalte gel
to direct zeolite units toward the formation of MFI framework [8].
However, complexity in tuning the suitable Si-to-Al and Na-to-Si
ratios, providing a precise control over synthesis parameters
including types of raw materials, mixing, aging and crystallization
conditions to obtain pure ZSM-5 phase, inhibiting any impurity
formation such as mordenite zeolite, low product yield and low
reproducibility restricted its evolution [9].
Synthesis of ZSM-5 applying high silica zeolite seeds (silicalite-1)
as cores and an integrated enriched aluminum shell without using
any extra template is a cost-effective route which has been under
spotlight in last few years. Silicalite-1 is a microporous polymorph
silica isostructural with the MFI structure consisting of eight five-
membered oxygen rings. Although the role of seeds are not
thoroughly comprehended but it is believed that silicalite-1 seeds
with similar framework and common composite building units of
ZSM-5 target zeolite are served as structure directing agent. In fact
seedsprovideasurfacewithTOHunitsonwhichformedzeoliteunits
can attach, pile up and grow [10]. According to Cundy and Cox seeds
* Corresponding author. Fax: +98 21 66957784.
E-mail address: shfatemi@ut.ac.ir (S. Fatemi).
http://dx.doi.org/10.1016/j.materresbull.2015.01.062
0025-5408/ã 2015 Published by Elsevier Ltd.

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M. Razavian et al. / Materials Research Bulletin 65 (2015) 253–259
behavior can be categorized in different ways [11]. Seeds can remain
inert and solely act as pure cores on which zeolite units can deposit
andgroworpartiallydissolveandleadtothesecondarynucleation.It
seemsthatseedsmayactcomplicatedandplayroleinmorethanone
way. Therefore seed-assisted crystallization mechanism and intrin-
sic seed crystals and reactant gel interaction still require vast
research. On the whole, this technique can address the environmen-
tal problems arising from the calcination of template and other
volatilecompoundswhicharenoteco-friendlymaterials. Inaddition
to, this technique combines several advantages including increasing
the crystallization rate, shortening the crystallization time, boosting
the solid yield, minimizing the template consumption, narrowing
the crystal size distribution and suppressing the formation of
undesired phases.
Fig. 1. XRD patterns of prepared samples: (a) silicalite-1, (b) Seed-ZSM-5 and (c)
Ref-ZSM-5.
Effect of seeding step in ZSM-5 crystallization process was
examined by introducing ZSM-5 crystals and pre-fabricated
silicalite-1 nanoseeds into the original self-induction system
[12]. Adoption of silicalite-1 nanoseeds had much more promi-
nent effect which led to the formation of relatively small particles
with promoted catalytic performance in benzene alkylation and
xylene isomerization. Majano et al. [13] investigated the influence
of diverse parameters like seed content and seed pretreatment in
the synthesis of Na-ZSM-5 using seed-induced crystallization
approach. Ren et al. [14] proposed a method based on seed surface
crystallization method for the rapid and controllable synthesis of
ZSM-5 zeolite with core–shell structure. Xue et al. reported the
successful synthesis of NH4-ZSM-5 zeolites with different Si/Al
ratio and size by simply adjusting the amount of seeding
suspension [15]. Addition of an organic additive and colloidal
silicalite-1 seeds to the initial synthesis mixture for the
fabrication of ZSM-5 were compared by Xu et al. [16]. More
uniform fine crystals were obtained through growing of
ZSM-5 shell around seeds that showed modified catalytic
performance in MTG reaction with tuned product distribution.
So far seeding synthesis trend in the literature has been mostly
around the usage of expensive templates such as tetrapropylam-
monium hydroxide (TPAOH) to achieve small uniform crystals but
the yield of the assay has been lowered [17–20]. In this paper we
produced ZSM-5 micro crystals through a seed surface crystalliza-
tion method employing TPABr-silicalite-1 core crystals. Our aim is
to propose an elaborate, efficient, facile and somehow cost-
effective strategy for the synthesis of ZSM-5 zeolite regarding high
solid product yield which would be applied for the large scale
production. Besides, the performance of this material as catalytic
support was evaluated in PDH reaction to convert propane to
highly valuable propylene as one of the most important
petrochemical building blocks with high selectivity and the
performance of the prepared catalyst was compared with the
literature.
2. Experimental
2.1. Synthesis of zeolite
Silicalite-1 seeding suspension was prepared from the hydro-
lysis of silica sol (30%, Sigma) in the solution containing
tetrapropylammonium bromide (99%, Merck) and sodium hydrox-
ide (98%, Merck). After complete hydrolysis, the obtained mixture
with molar composition of 1 SiO2:0.2 TPABr:0.03 Na2O:40 H2O was
hydrothermally treated at 150 ꢀC for 24 h. For characterization, a
portion of silicalite-1 product was isolated with centrifuging,
washing with distilled water and drying. As the second step, a
template-free gel was prepared by mixing sodium hydroxide,
sodium aluminate (54%, Riedel de Haen) as aluminum source and
same silica source to obtain a mixture with molar ratio of 60
SiO2:1 Al2O3:4.2 Na2O:1200 H2O. The specified amount of
silicalite-1 suspension with respect to 5% of total silica content
was added to the mixture directly without any extra purification.
Final crystallization step was performed at 170 ꢀC for 24 h. The
obtained product was recovered by centrifugation at 4000 rpm and
repeatedly re-suspended in deionized water until the sol exhibited
a pH value of about 9. Then it was dried and further calcined at
500 ꢀC for 6 h with heating ramping rate of 1 ꢀC/min.
Fig. 2. FESEM micrographs of silicalite-1 sample.

M. Razavian et al. / Materials Research Bulletin 65 (2015) 253–259
255
Fig. 3. SEM micrograph of ZSM-5 sample.
2.2. Preparation of the catalyst
spectrometer to determine metal content. FT-IR spectrum was
measured using KBr-diluted pellet on an IR spectrophotometer
(Bruker Tensor-27). N2 adsorption–desorption isotherms were
determined in a Micromeritics Tristar-3000 porosimeter. Textural
properties including total surface area, total pore volume,
micropore volume and average pore width were measured using
BET and t-plot methods. Before the surface area measurements,
samples were degassed at 300 ꢀC for 3 h.
The bimetallic PtSn/ZSM-5 catalyst was prepared by sequential
wet impregnation at 65 ꢀC with very dilute acidic solution of SnCl2
and aqueous solution of PtCl4 with 0.03 and 0.0133 M concen-
trations, respectively. Each impregnation step was followed by
drying and calcination at 500 ꢀC for 6 h. The nominal metal loading
was 0.5% of Pt and 1% of Sn.
2.3. Characterization
2.4. Catalytic evaluation
The X-ray powder diffraction (XRD) patterns of zeolites were
recorded at ambient temperature using a Philips Xpert system in
the 2
Field-emission electron microscope (Hitachi, S-4160) was used to
capture FE-SEM images. SEM images combined with energy-
dispersive X-ray analysis (EDAX) were obtained on a scanning
electron microscope (Philips, XL30ESEM, Netherlands) operated at
20 kV. Inductively coupled plasma atomic emission spectroscopy
measurement was performed on a Varian ICP-OES Vista-pro
Propane dehydrogenation was carried out in a fixed-bed tubular
quartz micro reactor with a propane and hydrogen mixture as feed.
0.5 gr catalyst was placed into the center of reactor and was heated
up to 500 ꢀC and maintained at this temperature under nitrogen
flow for 2 h. Reaction conditions including temperature, pressure,
WHSV and H2/C3H8 molar ratio were fixed on 600 ꢀC, 0.88 atm,
4 hꢁ1 and 0.8. Finally products were analyzed with an online YL-
6100 GC gas chromatograph equipped with a HID detector.
Propane conversion, product selectivitiy, and yield were defined
u a1 radiation (l= 1.54056 Å).
range of 5–50ꢀ using Cu-K

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M. Razavian et al. / Materials Research Bulletin 65 (2015) 253–259
Fig. 4. FT-IR pattern of seed-ZSM-5 sample.
Fig. 5. The N2 adsorption–desorption isotherms of (a) Seed-ZSM5 and (b)
Ref-ZSM-5.
as follows:
P
are two possibilities regarding crystallization in seed-induced
template-free systems in which the seed fragments grow
themselves or the products of the fragments dissolution provide
the nucleation units [1]. This concept would suggest that the
dissolution of these units occur in a specific manner. Besides, gel
restructuring at the core surfaces is influenced by the dissolved
species especially aluminum ones. Depending on the use of TPA+ or
Na+ ions, the Al distribution in ZSM-5 is enriched at the rim or
uniform throughout the crystal, respectively.
ni
3
Ci ꢁ Cpropane
i
out
P
ni
out
Propane conversion ¼
ꢂ 100
Ci
i
out
3
ni
Ci
out
Ci 3ꢁ Cpropane
Selectivity ¼P
ꢂ 100
ni
3
i
out
out
conversionð%Þ ꢂ selectivityð%Þ
FT-IR spectrum of seed-ZSM-5 product in a frequency range of
Yieldð%Þ ¼
400–4000 cmꢁ1 is shown in Fig. 4 to further identify the crystal
100
phase. Observed absorption bands at 453 cmꢁ1 (T—O (T
Si, Al)
¼
where i included all the components containing carbon atoms in
the exit gas stream, ni was the number of carbon atoms of
component i and Ci was its molar composition.
bending vibrations), 795 cmꢁ1 (external symmetric vibrations of
T—O—T groups), 1093 cmꢁ1 (internal asymmetric stretching
vibrations of Si—O—T linkages), 1224 cmꢁ1 (external linkages
between TO4 tetrahedra) and 3464 cmꢁ1 (stretching vibrations of
terminal Si—OH—Al hydroxyl groups) cmꢁ1 are typical character-
istic bands of highly siliceous MFI structure while the asymmetric
stretching vibration band at 548 cmꢁ1 has been assigned to
double-five ring structure. Detected bands in the region of 400–
1300 cmꢁ1 are due to the framework vibrations and give clues on
the skeletal features while stretching vibrations in 3400–
3800 cmꢁ1 region signify the existence of bridging hydroxyl acidic
sites located at the internal surfaces of zeolite. These hydroxyl
groups are active acidic centers that are responsible for acid-
catalyzed reactions. The weaker band around 3650 cmꢁ1 is
associated with the OH groups connected with the extra lattice
Al atoms that possess lower acid strength compared to structural
OH groups. These bands are in good agreement with those reported
in literature [21,22]. Since the most important impurity in the
pentasiles or amorphous silica phases does not show a band at
550 cmꢁ1, the optical density ratio of 550 over 450 cmꢁ1 is
examined to assess the formation of ZSM-5 and is named IR
crystallinity [23]. IR crystallinity of seed-ZSM-5 sample is 0.82
which indicates high degree of crystallinity, given that the optical
density ratio of pure pentasile is 0.8.
3. Results and discussion
3.1. Zeolite characterization
XRD spectrograms of all prepared samples along with
commercial HZSM-5 (provided by zeolyst company, Si/Al = 14) as
reference sample are illustrated in Fig. 1. All major peaks
representing MFI structure are found in XRD patterns but their
intensities are somehow different. Overall, it can be deduced that
seeding method employing TPABr-silicalite-1 crystals as MFI
structure directing agent was able to produce fully crystalline
pure ZSM-5 zeolite crystals.
SEM images of silicalite-1 and ZSM-5 samples are shown in
Figs. 2 and 3. Silicalite-1 sample showed very uniform polycrys-
talline particles with well-defined spherical shape without any
impurity or amorphous phase intergrowth with mean size of about
m. ZSM-5 particles had broader crystal size distribution in
comparison with seed particles ranging from 2 to 7 m with
predominent size of 3 to 4 m. One reason might be attributed to
the heterogenous silicalite-1 crystals dissolution in gel which
could act as primary units that grow and aggregate to zeolite
crystals eventually. Another reason can be assigned to unequal
aluminum distribution that consequently led to the formation of
zeolite crystals with different thickness and level of integrity. There
4
m
m
m
Textural properties of both ZSM-5 zeolite samples and
extracted diffraction parameters are summarized in Table 1.
Seed-ZSM-5 sample possess higher crystallinity and BET surface
Table 1
Physicochemical characteristics of ZSM-5 samples.
Sample
Relative crystallinity (%)a SEM size ( m) Scherrer crystal size (nm) SBET (m2/gr) Sext (m2/gr) Pore volume (cm3/gr) Average pore width (nm)c
m
XRD
Seed-ZSM-5 171
I550/I450
0.82
–
2.4–7.4
2–20b
51.52
58.62
342
237
121
24
0.16
0.12
1.9
2.2
Ref-ZSM-5
100
a
Relative crystallinity of the samples were calculated based on the three main characteristic peaks of MFI type zeolite at Bragg angles of 2
Particles were observed on the optical microscope.
Desorption average pore width (4 V/A by BET).
u
= 22–24ꢀ.
b
c

M. Razavian et al. / Materials Research Bulletin 65 (2015) 253–259
257
Table 2
zeolite product from consecutive synthesis procedure can be
produced using same molar composition in one-step synthesis
procedure considering same solid yield.
Comparison of obtainable solid product from single and double step synthesis
trends.
Solid yield (%)
88
Solid product in
one step (gr)
Solid product in
two step (gr)
The presence of Si, Al and Na basic elements and uniform
distribution of very small Pt and Sn-nanoclusters are confirmed by
EDX analysis along with elemental mapping (Fig. 6a–g). Addition-
ally, the quantitative elemental analysis of ZSM-5 catalyst is
described in the table, inside the spectral figure. The Si/Al ratio of
the final product is 17.44 which is lower than Si/Al ratio of the shell
starting synthesis gel (Si/Al = 30) indicating the preferential
insertion of Al atoms into zeolite framework. Moreover, the
incorporation of metals was measured 0.502 and 1.03% as Pt and Sn
content by ICP analysis. It can be concluded that metallic
compounds were successfully loaded on the support by the
employed sequential impregnation method.
Seed-ZSM-5
10
106
area comparing with Ref-ZSM-5 sample. Its higher pore volume is
related to more or wider pore mouths existing in the product
which explains the higher value of external surface area. This
concept is confirmed with nitrogen adsorption–desorption iso-
therms which are demonstrated in Fig. 5. High pore volume is an
effective catalyst specification that is needed for achieving a good
catalytic performance in a chemical reaction. High external surface
area exposes more entrances and diffusion paths to the reactants to
reach the active sites existing in the micropores more easily.
Moreover, high solid product yield is an important factor
regarding scaling up issue and reflects the success of synthesis
technique in comparison with other one-pot synthesis trends. It is
worth mentioning that in our procedure more than 100 gr of
zeolite can be synthesized employing silicalite-1 suspension
containing 5 gr of tetrapropylammonium bromide; a tremendous
quantity that cannot be obtained employing conventional one-step
procedures [24–26]. According toTable 2, around 10% of obtainable
3.2. Catalytic performance
Product selectivity vs. propane conversion is illustrated in Fig. 7
which is a complete experimental scheme of propane dehydroge-
nation over PtSn/ZSM-5 catalyst. The decreasing trend of propyl-
ene selectivity with conversion is because of protolytic cracking
(C—C bond dissociation) of propylene to lighter compounds, i.e.
methane, ethane and ethylene. It is assumed that propane
dehydrogenation proceed through carbenium ion intermediates
Fig. 6. (a) SEM micrograph of seed-ZSM-5, (b)–(f) represent the elemental mapping (Si, Al, Na, Pt and Sn) of the zeolite and (g) corresponding EDAX spectrum of the zeolite
along with weight and atomic percentages.

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M. Razavian et al. / Materials Research Bulletin 65 (2015) 253–259
Fig. 8. OPE-propylene and total olefin yields as a function of propane conversion.
Fig. 7. Propylene selectivity as
a function of propane conversion. (Reaction
conditions: 600 ꢀC, H2/C3 = 0.8 (molar ratio), WHSV of propane is 4 hꢁ1.)
Table 3
The catalytic performance of PtSn/ZSM-5 catalyst (reaction conditions: T = 600 ꢀC, catalyst weight = 0.5 gr, WHSV = 4 hꢁ1 H2/C3 = 0.8).
Conversion (%)
Selectivity (%)
Methane
Propylene yield (%)
Stabilitya
Ethane
Ethylene
Propylene
1b
42.2
2c
30.9
1
4.6
2
6.9
1
5.1
2
4
1
4.6
2
10.5
1
85.7
2
78.6
2
24.3
2
0.73
a
Stability = Xf/X0.
1 represents half an hour after reaction.
2 represents 7 h after reaction.
b
c
(C3+) (Olah’s monomolecular mechanism) which can further crack
composition and structural characteristics influence acidity
amount and consequently type and strength of produced
active sites. These structural characteristics are mostly related
to proton liability which depends on the angle formed between
two adjacent tetrahedra linking together at the oxygen carrying
a proton. Therefore, the importance of synthesis procedure
in determination of final features of made zeolite is an
indisputable fact.
According to the information mentioned above, it seems that
high initial activity of synthesized catalyst can be assigned to the
existence of large number of active sites on the outer surface of
catalyst. Substitution of aluminum with silicon atoms in shell
texture creates lots of acidic sites hosting metal atoms to construct
dehydrogenation active centers. Besides, reliable stable selectivity
toward light olefins, prominent selectivity toward propylene and
high stability factor of designated ZSM-5 catalyst confirm the
success of catalyst formulation procedure in declining coke
formation and deactivation phenomenon in which active sites
and micropores are covered with formed coke and heavy
carbonaceous products. It seems that piled up zeolite units
outfitted stable cores and improved catalyst stability parameter
which was comparable with other represented values that are
directly assigned to the rate of deactivation
to light hydrocarbons over support acidic centers by recombining
+
an alkyl moiety with the initial formed C3 carbenium ions,
lowering propylene selectivity [27,28]. In other words, cracking
phenomenon plays a significant role in reaction mechanism which
seems to get more dominant with time on stream. At this juncture,
it is interesting to mention the considerable stable total light
olefins selectivity as a privilege to prove the efficiency of obtained
catalyst in producing key petrochemical components, i.e. ethylene
and propylene in the present economy context. The efficiency of
synthesized catalyst in the propane dehydrogenation reaction can
be studied using olefins performance envelop (OPE) which is
shown in Fig. 8.
Table 3 lists the obtained data of propane dehydrogenation
catalytic test using prepared PtSn/ZSM-5 catalyst after 0.5 and 7 h
of reaction as initial and final points, respectively. Table 4
summarizes different bi-metallic ZSM-5 catalytic PDH results
from open literature to be able to compare the efficiency of our
synthesis procedure. Since acidic sites provide oxygen onto the
surface of metal clusters in proton transfer reactions to form
oxidized metal species as reactive active sites, support type
and structure are counted as key factors explaining catalytic
behavior of zeolites. On the other hand, support chemical
Table 4
Comparison of catalytic performance of different catalysts after 5 h time on stream (P = 0.1 Mpa H2/C3 = 0.25).
Catalyst type
Test conditions
Conversion (%)
Propene selectivity (%)
Propene yield (%)
Stability
Ref.
PtSn/ZSM-5
PtSn/ZSM-5
PtSn/ZSM-5
PtSn/ZSM-5
PtSn/ZSM-5
PtSn/ZSM-5
PtSn/ZSM-5
T = 590 ꢀC, WHSV = 3 hꢁꢁ11
27
22.9
32
25
25
73.5
49.1
37
52
42
20.6
11.2
11.8
13
10.5
10.7
26.6
0.78
0.66
0.64
0.69
0.76
0.6
[29]
[30]
[31]
[32]
[33]
[34]
This work
T = 590 ꢀC, WHSV = 3 h
T = 590 ꢀC, WHSV = 3 hꢁꢁ11
T = 590 ꢀC, WHSV = 3 h
T = 600 ꢀC, WHSV = 5.6 hꢁꢁ11
T = 600 ꢀC, WHSV = 5.9 h
T = 600 ꢀC, WHSV = 4 hꢁ1, H2/C3 = 0.8
25
33.1
43
80.3
0.79

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