Synthesis and catalytic performance of zeolite-Y supported on silicon carbide in n-heptane cracking

Article abstract of DOI:10.1016/j.apcata.2020.117866

In this work, we demonstrate a facile approach for the synthesis of zeolite-Y crystals (size, ca. ～400 nm) supported on silicon carbide (SiC) with the assistance of the cationic template (polydiallyldimethylammonium chloride, PDDA). The polymeric cationic template used to treat SiC particles induces a positive charge on SiC surface which electrostatically attracts negatively charged aluminosilicate seeds and promotes the growth of zeolite (ZY) particles over SiC, thus leading to the formation of stable ZY?SiC supported catalysts. The supported ZY catalysts with different weight ratio of ZY and SiC were synthesized and characterized by various techniques such as XRD, SEM, SEM-EDX, SEM-mapping, TEM, STEM, FT-IR, ²⁷Al MAS NMR and N₂ sorption. The characterization of the supported ZY catalysts suggests the uniform growth of ZY particles over SiC together with the creation of hierarchical micro-mesopores assembly. In the catalytic cracking of n-heptane, the catalyst ZY?SiC-50 displayed a remarkable improvement in reaction rate when compared to commercial zeolite-Y (CBV-600) amounting to 3.5 folds enhancement. Interestingly, the light olefins yield is also substantially improved. At WHSV of 8 h^?1 and 475 °C, the highest light olefin yield (24–36 %) was achieved over ZY?SiC-50 whereas the reference catalyst, CBV-600 produced lower light olefins yield (7–17 %). Moreover, the supported ZY catalyst exhibited less deactivation rates. This improved performance is attributed to the hierarchical micro-mesopores assembly created by the homogeneous dispersion of zeolite crystals on SiC which offers fast diffusion pathways for the reactants and enhanced accessibility to active sites thus leading to higher observed reaction rates and fast diffusion of products thus minimizing the occurrence of side reactions.

Full text of DOI:10.1016/j.apcata.2020.117866

Journal Pre-proof
Synthesis and catalytic performance of zeolite-Y supported on silicon
carbide in n-heptane cracking
Aasif A. Dabbawala (Investigation) (Methodology)
(Conceptualization) (Formal analysis) (Writing - original draft),
Balasubramanian V. Vaithilingam (Writing - review and editing)
(Data curation) (Visualization) (Validation), Hemant mittal (Formal
analysis) (Data curation), Yasser Al Wahedi (Writing - review and
editing) (Software) (Supervision) (Project administration)
(Resources), Shairoz khan (Investigation) (Data curation), Tony
Joseph (Investigation) (Methodology) (Data curation), Gnana
Singaravel (Supervision) (Visualization) (Validation), Stephane
Morin (Supervision) (Visualization) (Validation), Mikael Berthod
(Supervision) (Visualization) (Validation), Saeed M. Alhassan
(Conceptualization)<ce:contributor-role>Writing - review and
editing) (Funding acquisition) (Supervision) (Project administration)
(Resources)
PII:
S0926-860X(20)30459-2
DOI:
https://doi.org/10.1016/j.apcata.2020.117866
APCATA 117866
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
8 June 2020
17 September 2020
25 September 2020
Please cite this article as: Dabbawala AA, Vaithilingam BV, mittal H, Wahedi YA, khan S,
Joseph T, Singaravel G, Morin S, Berthod M, Alhassan SM, Synthesis and catalytic
performance of zeolite-Y supported on silicon carbide in n-heptane cracking, Applied
Catalysis A, General (2020), doi: https://doi.org/10.1016/j.apcata.2020.117866

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Synthesis and catalytic performance of zeolite-Y supported on silicon carbide in n-
heptane cracking
Aasif A. Dabbawalaa, Balasubramanian V. Vaithilingamb, Hemant mittala, Yasser Al Wahedia,c*,
Shairoz khana, Tony Josephb, Gnana Singaravelb, Stephane Morinb, Mikael Berthodb,
M. Alhassana*
Saeed
aChemical engineering department, Khalifa University of Science and Technology,
Campus, Abu Dhabi, UAE
SAN
bADNOC Refining Innovation Center, Abu Dhabi, UAE
cCenter for Catalysis and Separation, Khalifa University, P.O. Box 127788, Abu Dhabi, United
Arab Emirates
Corresponding E-mail: yasser.alwahedi@ku.ac.ae, saeed.alkhazraji@ku.ac.ae
Graphical abstract
1

Research Highlights
 Synthesized zeolite-Y supported on silicon carbide (ZY@SiC) using cationic template
 Cationic template played key role to promote growth of ZY particles on SiC surface
 ZY particles uniformly grown on SiC surface and create hierarchical pores assembly
 ZY@SiC-50 displayed a remarkable enhancement in n-heptane cracking reaction rate
 ZY@SiC-50 exhibited improvement in light olefins yield
Abstract
In this work, we demonstrate a facile approach for the synthesis of zeolite-Y crystals (size, ca.
400 nm) supported on silicon carbide (SiC) with the assistance of the cationic template
(polydiallyldimethylammonium chloride, PDDA). The polymeric cationic template used to treat
SiC particles induces a positive charge on SiC surface which electrostatically attracts negatively
charged aluminosilicate seeds and promotes the growth of zeolite (ZY) particles over SiC, thus
leading to the formation of stable ZY@SiC supported catalysts. The supported ZY catalysts with
2

different weight ratio of ZY and SiC were synthesized and characterized by various techniques
27
such as XRD, SEM, SEM-EDX, SEM-mapping, TEM, STEM, FT-IR, Al MAS NMR and N2
sorption. The characterization of the supported ZY catalysts suggests the uniform growth of ZY
particles over SiC together with the creation of hierarchical micro-mesopores assembly. In the
catalytic cracking of n-heptane, the catalyst ZY@SiC-50 displayed a remarkable improvement in
reaction rate when compared to commercial zeolite-Y (CBV-600) amounting to 3.5 folds
enhancement. Interestingly, the light olefins yield is also substantially improved. At WHSV of 8
h-1 and 475 °C, the highest light olefin yield (24-36%) was achieved over ZY@SiC-50 whereas
the reference catalyst, CBV-600 produced lower light olefins yield (7-17%). Moreover, the
supported ZY catalyst exhibited less deactivation rates. This improved performance is attributed
to the hierarchical micro-mesopores assembly created by the homogeneous dispersion of zeolite
crystals on SiC which offers fast diffusion pathways for the reactants and enhanced accessibility
to active sites thus leading to higher observed reaction rates and fast diffusion of products thus
minimizing the occurrence of side reactions.
Keywords: Zeolite-Y, SiC, Supported catalyst, n-heptane, Cracking
1. Introduction
Catalytic cracking of light alkanes (such as n-hexane, n-heptane) has been recognized as
one of the most imperative method in oil-refining industry to produce light olefins as it provides
flexibility to tune products distribution and considered a more energy-efficient way in comparison
to traditional thermal cracking [1, 2]. Light olefins (generally refer to ethylene propylene and
butylenes) which hold the utmost importance in downstream chemical industry due to the wide
spectrum of products that are manufactured from them (e.g. polyethylene, polypropylene and
alkylbenzenes) [1, 2]. Increasing the light olefins yield via selective catalytic cracking of heavy oil
3

fractions is desired due to their economic benefit for oil-refining industry [1-4]. Zeolites are
crystalline and microporous aluminosilicate materials consisting of a three-dimensional
framework constructed by interconnected SiO4 and AlO4 tetrahedra [5-7]. Several zeolites earn
importance in a range of applications which includes adsorption, gas separation, ion exchange and
catalysis [5-15]. One of the significant applications of zeolite perceived in petro-chemical industry
is catalytic cracking [16-18]. In particular, Zeolite Y, a member of faujasite family (Si/Al atomic
ratio > 1.5) possesses high surface area, pore volume, high thermal stability and a micropores
channel of 0.74 nm in diameter arising from the assembly of the sodalite cages and supercages
[19]. Zeolite Y has been deployed in the area of Fluid Catalytic Cracking (FCC) for decades due
to the intrinsic structural/surface properties and the presence of high concentration of acid sites
[16-21].Furthermore, the microporous structure of zeolite Y attributed to its well-defined pore size
leads to molecular shape selectivity. However, the narrow microporous network in zeolite Y with
large crystal size induces diffusion limitations for bulky molecules. At such conditions the kinetics
of the process becomes mass transfer limited. Furthermore, the mass transfer limitations force the
products to engage into undesirable secondary reactions. Those reactions are the primary cause for
coking and subsequent deactivation of the catalyst. Therefore, the approach that alleviates the mass
transfer limitation by the optimization/improvement of surface and porous properties thus
enhancing acid site accessibility, boosting catalyst activity and minimizing side reactions is highly
desirable.
In order to overcome mass transfer limitations in microporous zeolites, the research has
been directed towards the synthesis of hierarchical zeolite materials via different concepts [22-36].
For example, (i) the reduction of zeolite crystal size i.e. synthesis of nanozeolite thus creating inter-
crystalline mesoporosity, (ii) the introduction of mesoporosity in the zeolite framework (intra-
crystalline porosity) and (iii) the synthesis of hierarchically structured zeolite composite wherein
4

zeolite crystals are loaded on porous matrix. The nanosize zeolite crystals have higher surface area
and higher inter-crystalline mesopores volume compared to that of micro sized zeolites thus
facilitating mass transfer [26-30]. However, the direct use nanozeolite is still challenging because
of their lack of crystalline stability, ease of aggregation and the need of expensive organic
templates [28-30]. Another approach that addressed the aforementioned issues is to grow zeolite
particles on support matrices such as Al2O3, TiO2, TUD-1 and Mesosilica to form hierarchically
structured composites. Such composites possess microporosity and mesoporosity in close
proximity which enhances diffusion rates and hence catalytic performance [31-34]. To ease heat
transfer and mechanical stability, thermo-conductive support such as silicon carbide (SiC) can be
superior alternative to conventional support materials especially in exothermic or endothermic
reactions [35-47]. Silicon carbides (SiC) in the forms of alpha (α) and beta (β) polymorphs are
popular manufacturing materials available with distinct structure, shape and porosity and have
been utilized as supports for catalysts due to its high mechanical strength and thermal conductivity,
high chemical and thermal resistance [35-41]. Mostly, the high surface area microporous zeolites
have been grown on surface of SiC support with the intention of optimizing their catalytic
performances [39-47]. Winé et al. reported synthesis of composites comprised of zeolite beta and
SiC via hydrothermal method [39]. The resulting zeolite beta supported on SiC composites were
studied as catalyst for Friedel-Crafts benzoylation reaction in liquid-phase as well as in fixed bed
configuration. SiC supported beta zeolite exhibited high catalytic activity and found less sensitive
to deactivation as compared to unsupported commercial beta zeolite [39-41]. Ivanova and co-
workers prepared structured composites of ZSM-5 and β-SiC and assessed their catalyst
performance in methanol to olefins reaction. The resulted ZSM-5 coated β-SiC composite also
showed improvement in activity/selectivity when compared to conventional ZSM-5 power
counterpart [42,43]. The SiC based composite catalysts displayed higher stability and activity.
5

However, in the literature, most of studies focused on growing ZSM-5 or Beta zeolite on SiC
support for methanol to olefins and Friedel-Crafts reactions [39-46]. To the best of our knowledge,
studies on the growth of zeolite-Y (ZY) particles on SiC support especially for cracking
applications are lacking. Moreover, in most of reported methods, the SiC supported zeolite
catalysts were prepared by multiple steps where the hydrothermal crystallization of zeolite was
performed two/three times to achieve sufficient loading of zeolite crystals on SiC support [43-47].
In addition, the high calcination pretreatment (~ 900 °C) of SiC is crucial for the growth of zeolite
crystals over SiC surface. In this context and encouraging the advantages of SiC support, we
focused to fabricate SiC supported catalyst comprises of ZY and SiC in straightforward
methodology. In the present work, we demonstrate a facile in situ approach for the synthesis of
zeolite-Y supported on silicon carbide assembly and investigated their catalytic performance for
n-heptane cracking. The SiC supported zeolite-Y catalysts were synthesized in one step under
hydrothermal condition using SiC particles treated with polymeric cationic template
(polydiallyldimethylammonium chloride, PDDA). The treatment of SiC particles with cationic
templates generates positive charges on the surface of SiC which attract negatively charged
preliminary structure units of ZY seeds through electrostatic interactions. The quaternary
ammonium moieties of the cationic template act as a conventional Structure Directing Agent
(SDA), tetra-methyl ammonium salt, to guide the growth of ZY crystals over the SiC surface
(ZY@SiC). The present method ensures the uniform growth of ZY particles on the surface of SiC
and the formation of stable ZY@SiC assembly. The proposed growth mechanism of ZY crystals
on cationic template treated SiC surface is presented in Fig. 1.
6

Fig. 1. In-situ approach for the synthesis of zeolite-Y supported on silicon carbide (zeolite-
Y@SiC) and its catalytic application in n-heptane cracking.
The synthesized SiC supported zeolite-Y (ZY@SiC) catalysts were characterized by XRD, SEM,
EDX, elemental mapping, TEM, STEM, N2 sorption, NMR and FT-IR techniques. Finally, all
these catalysts with varying weight ratio of ZY to SiC were evaluated for n-heptane cracking. The
catalytic activity of ZY@SiC catalysts was investigated at different temperatures and
benchmarked against the activity of commercial zeolite-Y (CBV-600) and the physically mixed
catalyst (ZY/SiC).
2. Experimental
2.1. Material
The support material, β-SiC granules with particle sizes ranging from 200 to 500 µm were
purchased
from
SICAT
SARL
(France).
Polymeric
cationic
template,
Poly(diallydimethylammonium chloride, PDDA (20 wt. % in H2O, Mw ~ 200,000) solution was
purchased from Sigma-Aldrich chemicals. Other reagents used in ZY synthesis were sodium
7

hydroxide (NaOH pellets EMPLURA®, >99% purity) from Merck chemicals, sodium aluminate
(50-55%, Al2O3, 40-45% Na2O) and Ludox (HS 40% colloidal silica) from Sigma-Aldrich
chemicals. All the above reagents were used as received without further purification.
2.2. Synthesis of zeolite-Y and SiC supported zeolite-Y catalysts
Zeolite-Y sample (ZY, particles size, approx. 400 nm) was hydrothermally synthesized
without organic template as reported by Awala et al. with slight modification in precursors
composition [48]. The dense-gel method was used to prepare ZY possessing particles size of 400
nm. The molar composition of the initial gel was set to 8 Na2O: 1 Al2O3: 14 SiO2: 410 H2O. In a
typical synthesis, 2.8 g of NaOH is dissolved in 31.5 g of H2O at room temperature and then 0.96
g of sodium aluminate (NaAlO3) is added under stirring until a clear solution is obtained. Finally,
7.6 g of Ludox silica-HS (40%, colloidal silica) is added dropwise under vigorous stirring to the
sodium aluminate solution. The addition of the silica precursor, Ludox HS-40 to basic sodium
aluminate solution produces dense aluminosilicate gel which is kept for aging for 24 h at RT. Then,
the aged aluminosilicate gel is kept for hydrothermal crystallization at 100 °C for 12 h. Upon
completion of hydrothermal treatment, the material was centrifuged and washed several times with
deionized water until the pH reaches to ~7-8 and then dried in hot air oven at 120 °C for 12 h.
For the synthesis of ZY@SiC catalysts, 1.25 g PDDA (20% solution in water, MW ~
200,000) is added to a mixture of SiC granules (5 g) and 25 mL ethanol. The resulting mixture is
stirred for 1 day to allow uniform coverage of PDDA on the SiC surface and then the ethanol was
removed by rotary evaporator to obtain PDDA treated SiC (termed as PD-SiC). The resulting PD-
SiC is dried in oven at 80 °C for 12 h. The aluminosilicate gel was synthesized as mentioned above
with composition of 8 Na2O: 1 Al2O3: 14 SiO2: 410 H2O and then cationic template treated SiC
particles were added to the aluminosilicate gel that was aged for 20 h (at this stage zeolite seeds
8

are developed) then aging is continued for another 4 h. Finally, the whole mixture was placed in
an autoclave and kept at 100 °C for the duration of 12 h for hydrothermal crystallization. Likewise,
various ZY@SiC samples were synthesized by varying the PD-SiC amount. The ZY@SiC samples
containing 30, 50 and 70 wt% PD-SiC are denoted as ZY@PD-SiC-30, ZY@PD-SiC-50 and
ZY@PD-SiC-70 respectively (Table S1). All the synthesized ZY@SiC samples were calcined at
500 °C (for 5 h) in the presence of air. For catalyst testing, the Na+ form of synthesized supported
catalysts were ion exchanged with NH4Cl and calcined at 450 °C to obtain H+ form of ZY catalysts.
The calcined and ion-exchange catalysts are denoted as ZY@SiC-30, ZY@/SiC-50 and ZY@/SiC-
70. For comparison purpose, one sample is prepared without template treatment on SiC, which
termed as ZY@WT-SiC-50 (WT-without template). In addition, the physically mixed catalyst
ZY/SiC-50 is also prepared by simple grinding of ZY and SiC using 1:1 weight ratio.
2.3. Material Characterization
SEM (scanning electron microscopy) images of ZY, β-SiC and ZY@SiC samples were
acquired on FEI Quanta 250 (FEG) instrument which is equipped with energy dispersion
spectrometer-EDX. SEM images of samples were collected at 3 and 5 kV to minimize charging.
XRD (X-Ray Diffraction) patterns for ZY, β-SiC and ZY@SiC samples were acquired by
Panalytical X’Pert PRO powder diffractometer using Cu Kα radiation, λ = 1.5406 ꢀ, 45 kV, 40
mA. X-Ray diffraction patterns of samples were collected in the range of 3 to 60° 2θ scale with
step size, 0.02°. FT-IR spectra for ZY and ZY@SiC samples were collected from Bruker Vertex
80v FT-IR in the range from 4000 to 400 cm-1. TEM-Transmission Electron Microscopy images
for ZY and ZY@SiC samples were collected from FEI Tecnai G2 F30 TEM instrument operating
at 200 kV acceleration voltage. For TEM-imaging, a small amount of sample is suspended in
ethanol and sonicated for few minutes. Then, a drop of the resulting suspension is applied to a
9

lacey carbon coated copper grid (Ted Pella Inc.). N2 sorption (adsorption-desorption) isotherms,
surface area and pore volume of samples were acquired from Micromeritics 3Flex surface area
and porosimetry analyzer (Micromeritics Instruments Corporation, USA) using N2 gas of ultra-
high purity (99.99%) at 77 K. All the samples were degassed at 250 °C for 12 h prior to N2 sorption
measurements.
2.4. Catalytic measurement
The catalytic cracking of n-heptane was performed in a fixed bed reactor (inner diameter,
9 mm) at atmospheric pressure. For each catalytic cracking test, 0.5 g of the sample was loaded to
reactor and the sample was sandwiched between two layers of quartz wool. The cracking reaction
was performed at the temperatures of 425, 450 and 475 °C with WHSV-weight hourly space
velocity of 8 h-1. N2 (Nitrogen) was used as carrier gas and mixed with n-heptane. The gaseous
mixture was then passed into the fix bed reactor with feed (n-heptane) flow rate of 0.1 mL/min
and N2 flow rate of 20 mL/min. The samples were collected at specific time of intervals and
analyzed using GC 7890A, 8-column RGA with FID detector. The major products identified were
ethylene, ethane, propylene, propane, n-butane, isobutane, 1-butylene and isobutylene. The
conversion of n-heptane and products yield were calculated on molar basis and represented in this
manuscript.
3. Results and Discussions
3.1. Structural and morphology assessment of Zeolite-Y and SiC supported zeolite-Y
XRD patterns of ZY, β-SiC, PD-β-SiC and ZY@SiC samples prepared using cationic
template treated SiC are presented in Fig. 2. Whereas, XRD pattern and SEM image of ZY@WT-
SiC-50 synthesized with non-treated SiC is shown in (Fig. S1). From the XRD pattern and SEM
image of ZY@WT-SiC-50, it can be seen that the FAU phase formation is affected i.e. additional
10

diffraction peaks are observed with FAU phase and ZY crystals grew separately from the SiC. The
separate formation of ZY crystals demonstrates no interaction between SiC and ZY crystals due
to the absence of the cationic template on SiC surface. On the other hand, the XRD patterns of
ZY@SiC samples display only the characteristic peaks of FAU phase along with β-SiC. This is
evident by the presence of the distinctive diffraction peaks at 6.2 o, 10.2 o, 11.9 o, 15.7 o, 23.6 o,
26.97 o, 31.4o (2θ) belonging to the FAU phase and additional peaks at 36o, 41o, 60o and 71o (2θ)
belonging to the β-SiC phase. The addition of the cationic template-treated SiC did not result into
the formation of extra crystalline phases. SEM images of ZY@PD-SiC-50 (Fig. S2) suggests the
creation of a uniformly distributed coating of ZY crystals on the SiC surface indicating the
imperative role of PDDA for uniform growth of ZY on SiC. The effects of the support surface and
its modification for anchoring zeolite crystals have been studied by many researchers [35,36, 39-
44]. For instance, Candamano et al. reported the growth of ZY particles on cordierite ceramic
support [35]. The authors reported partial dissolution of the support during the crystallization
process which allowed the chemical anchorage of zeolite particles on the surface of the support.
In another study, Wine and co-workers anchored BEA zeolite particles on calcined β-SiC. The
oxide layer developed on the support surface during calcination enables for anchoring the zeolite
particles via Si-O-Si bond [39-41]. Ivanova et al. also proposed a similar mechanism for the
coating of ZSM-5 on β-SiC support whereby the nanoscopic SiO2 layer at the support surface play
the anchorage role [42,43]. In the present study, when uncalcined non-treated SiC particles were
used for the supported catalyst preparation, the growth of ZY crystals on the SiC surface was not
observed, which probably due to lack of binding sites on the SiC surface. In the case of cationic
template treated SiC, the positive charge generated on the SiC surface attracts primary crystals
seeds of ZY and thereby directs the growth of the ZY crystals on the surface of β-SiC. The cationic
template’s polymeric nature enhances the adsorption affinity of template on the SiC surface which
11

thereby resulting in assembly comprised of uniformly grown ZY particles on SiC surface. The
XRD patterns of the synthesized samples, ZY@PD-SiC-30, ZY@PD-SiC-50 and ZY@PD-SiC-
70 are shown in Fig. 2.
(B)
Fig. 2. XRD patterns of SiC, PD-SiC, ZY (A); ZY@PD-SiC samples synthesized with different
weight content of PD-SiC (B).
All the ZY@PD-SiC samples display the characteristic peaks of FAU phase and β-SiC which
indicates that the variation of the PD-SiC amount did not disrupt the formation of the pure FAU
phase. Moreover, the common impurities that accompany FAU phase growth such as Zeolite-
A/Zeolite P were not observed.
The SEM images of β-SiC, PD-SiC, ZY and ZY@PD-SiC are presented in Fig. 3. SEM-
EDX and elemental mapping for ZY@PD-SiC-50 are also presented in Fig. S3. An irregular
morphology with coarse surface can be seen for both β-SiC and PD-SiC particles. Whereas, ZY
particles display the typical cubic-octahedral morphology of FAU phase with particle sizes ranging
from 400 to 500 nm (Fig. 3d). The SEM images of ZY@PD-SiC samples provide clear evidence
of the growth of ZY crystals on PD-SiC particles (Fig. 3e-3l). Furthermore, the cubic-shape
morphology of ZY is retained in all the samples irrespective of the ZY contents. The SEM-EDX
12

of the ZY@PD-SiC-50 confirms the presence of Si, Al, O, Na and C elements. The SEM-elemental
map of the sample, ZY@PD-SiC-50 displays a uniform distribution of Si, C, Al, O and Na
elements over the entire selected region (Fig. S3). It is also observed that the silica density
increases considerably while the carbon density decreases in the area where ZY particles grew
over the PD-SiC surface. This further confirms the uniform growth of ZY particles on β-SiC
surface (Fig. 3). Usually, the alumino-silicate sol starts to produces crystals nuclei (seeds) of ZY
after aging for 20 h [49]. At this stage, the addition of the cationic template treated SiC (PD-SiC)
to the alumino-silicate sol attracts seeds of ZY possessing negative surface charges (generated due
-
to the isomorphous substitution of SiO4 by AlO4 ). Therefore, the PD-SiC particles are surround
by the seeding ZY crystals by virtue of electrostatic attraction. In consequence, the growth of the
ZY crystals occurs on the surface of SiC. Furthermore, the presence of the Si-OH groups in β-SiC
facilitates stable anchorage of ZY crystals via chemical bonding (Si-O-Si bond) during the
hydrothermal crystallization.
13

Fig. 3. SEM images of β-SiC (a), PD-SiC (b,c), ZY (d); ZY@PD-SiC-30 (e, f), ZY@PD-SiC-50
(g-j), ZY@PD-SiC-70 (k,l). Figures (f), (h),(l) and (j) are zoomed images of the areas enclosed by
the dashed line rectangles in Figures (e),(g),(k), and (i) respectively.
In ZY@PD-SiC samples possessing lower content of SiC, the independently grown ZY crystals
(not on the surface of SiC) are observed in large amounts due to the limited availability of SiC
particles. The increase of SiC content promotes better coverage of the zeolite crystals on SiC
surface. In order to achieve optimal ZY coverage of SiC surface (i.e. absence of independently
grown ZY), the optimal weigh content of SiC should be above 30%. This would provide material
with more catalytically active surface sites to be easily available for reactants. TEM images of ZY,
β-SiC and ZY@PD-SiC samples are depicted in Fig. 4 with STEM image of ZY-PD-SiC-50. The
TEM images of ZY and β-SiC also display the typical cubic-octahedral morphology of ZY and
irregular spherical morphology of SiC particles. Whereas all ZY@PD-SiC samples exhibit the
14

interconnected ZY and SiC particles or SiC particles are homogeneously surrounded by ZY
crystals thus corroborating SEM observations. Furthermore, the STEM image of ZY-PD-SiC-50
indicates organized assembly of ZY and SiC (ZY@SiC).
Fig. 4. TEM images of ZY (a), β-SiC (b,c), ZY@PD-SiC-30 (d,e), ZY@PD-SiC-50 (f,g),
ZY@PD-SiC-70 (h,i), STEM image of ZY@PD-SiC-50 (j). Figures (c) and (i) are zoomed images
of the areas enclosed by the dashed line rectangles in Figures (b) and (h) respectively.
3.2. Spectral and textural assessment of Zeolite-Y and SiC supported zeolite-Y
The FT-IR spectra of ZY, SiC, PD-SiC and ZY@PD-SiC samples are presented in Fig. 5.
In the ZY sample, the absorption bands at 566, 689 and 774 cm-1 correspond to the double six-
member rings (D6R) and the symmetrical stretching vibration of Si-O-Al within zeolite framework
[50]. The band at 454 cm-1 can be attributed to the tetrahedral TO4 units (T = Si or Al) internal
vibration. Overall, it evidences the formation of the FAU phase. The absence of the bands at 1440
15

and 1332 cm-1 suggests the complete dissolution of amorphous aluminosilicates and conversion to
the crystalline FAU phase during hydrothermal crystallization [50,51]. In the FT-IR spectra of the
SiC and PD-SiC samples, a strong absorption peak at 804 cm-1 and a weak absorption peak at 1090
cm-1 are also observed which can be ascribed to the stretching vibration peaks of Si–C and Si–O
bonds respectively. These peaks are the characteristic absorption peaks of β-SiC [52].
(C)
Fig. 5. FT-IR spectra of SiC, PD-SiC, ZY (A); ZY@PD-SiC samples synthesized with different
weight content of PD-SiC (B). 27Al MAS NMR spectra of ZY and ZY@SiC-50 (C).
16

Compared with the original SiC, PD-SiC displays additional absorption peaks at 2858 cm-1 and
2,934 cm-1 corresponding to the C-H stretching vibration of the polymeric alkyl chain. This is an
evidence of the presence of PDDA on the surface of SiC. The absence of the bands pertaining to
N-C stretching vibrations can be attributed to the low PDDA content on the SiC. The ZY@PD-
SiC samples synthesized using different PD-SiC content displays all the characteristic FAU and
SiC absorption bands albeit at a lesser intensity. Moreover, in all aforementioned samples, the
absorption bands at 1440 and 1332 cm-1 corresponding to amorphous aluminosilicates are absent
thus suggesting complete transformation of the aluminoslicates to FAU phase during the
27
27
hydrothermal crystallization step. Al MAS NMR spectra of ZY are presented in Fig. 5C. Al
MAS NMR spectrum of ZY shows one peak at around 61 ppm corresponding to the tetra-
coordinated framework aluminum ions [49,53]. The presence of only one peak in 27Al MAS NMR
spectrum of ZY indicates absence of extra-framework aluminum species in the ZY sample. In the
27
case of the sample (ZY@SiC-50), Al MAS NMR spectrum also showed only one peak at the
same position of ZY around 61 ppm indicating that the aluminum framework was not affected in
zeolite-Y in the ZY@SiC-50 sample (Fig. 5C). The types of surface acid sites in the zeolite
samples can be determined by pyridine FT-IR as pyridine is considered a selective alkaline probe
which interacts predominantly with surface acid sites. The FT-IR spectra (in the pyridine ring
stretching region) of ZY and ZY@SiC samples are presented in Fig. S4. ZY samples exhibit three
distinct peaks at 1444, 1489 and 1542 cm-1 corresponding to Lewis and Brønsted acid sites (Fig.
S4). The broad band centered at 1542 cm-1 is attributed to Brønsted acid sites [54]. In addition, the
broad band centered at 1444 cm-1 is attributed to Lewis acid sites [54]. The ZY@SiC samples also
display all three peaks related to Brønsted and Lewis acid sites similar to ZY but the intensity of
peak related to acid site varies with zeolite content present in SiC supported ZY catalysts as SiC
17

support is neutral material which did not influence on nature of acid sites pertaining to zeolite-Y.
Hence, in the ZY@SiC system, the zeolite particles are the only catalytically active component.
It is anticipated that the dispersion of ZY on SiC surface may result into a hierarchical
micro-mesopores assembly. To test this hypothesis, N2 adsorption-desorption experiments of ZY,
β-SiC and ZY@SiC samples were carried out. Nitrogen sorption isotherms and pore size
distributions of the ZY@SiC samples (HK for micropores and BJH for mesopores) are presented
in Fig. 6. Textural and surface properties of the ZY, β-SiC and ZY@SiC samples are presented in
Table 1.
Table 1 Surface and textural properties of β-SiC, ZY and ZY@SiC samples
a
b
c
SABET
PVtotal
PVmicro
PVmeso
Sext
HFd
Sample
(m2/g)
(cm3/g)
(cm3/g)
(cm3/g)
(m2/g)
β-SiC
19
0.124
0.003
0.30
0.19
0.14
0.09
0.121
17
27
24
23
19
0.022
0.030
0.040
0.045
0.041
ZY
848
489
376
275
0.31
0.23
0.19
0.15
0.01
0.04
0.05
0.06
ZY@SiC-30
ZY@SiC-50
ZY@SiC-70
a Determined at P/P0 near to unity, b t plot method, c PVtotal
d HF (hierarchy factor) = (Sext * PVmicro)/(SABET * PVtotal) [54].
̶ PVmicro.
Apparently, ZY exhibits type-I isotherm typically attributed to microporous materials (sharp N2
uptake occurring at low relative pressure (P/Po < 0.2). On the contrary, β-SiC particles exhibited
higher uptake only at high relative pressures. Furthermore, the pore volume of β-SiC is only 0.124
cm3/g. The absence of any evidence of significant uptake at low pressure range and the wide peak
centred at 50 nm in the pore size distribution curve assure the presence of large-sized pores on SiC
surface; a typical feature of macroporous material. All ZY@SiC samples possess higher BET
18

surface areas and larger pore volumes when compared to the β-SiC sample. This is due to the
deposition of the ZY particles on SiC. The presence of both micropores and meso/macropores on
the ZY@SiC samples is an evidence of the hierarchical micro-mesopores assembly.
Fig. 6. N2 isotherms of β-SiC, ZY and ZY@SiC samples (A) ; HK micropore size distribution of
ZY and ZY@SiC samples (B); BJH pore size distribution of β-SiC (C), ZY and ZY@SiC samples
(D). The N2 isotherms (in Fig. 6A) of samples ZY@SiC-70, ZY@SiC-50, ZY@SiC-30, and ZY
were vertically offset by 210, 150, 105 and 80 cc/g respectively.
According to N2 sorption (adsorption-desorption) isotherms of ZY@SiC samples, all the samples
display a combination of type I and type IV isotherms which indicates the coexistence of micro
and mesopores in the ZY@SiC. The isotherms of the ZY@SiC samples exhibit a sharp uptake at
19

lower pressure range (P/Po < 0.2) similar to ZY suggesting the presence of microporosity due to
the ZY particles. This is further corroborated by the existence of a micropore size distribution of
the ZY@SiC derived from Horvath-Kawazoe (HK) method (Fig. 6). The ZY@SiC samples also
exhibit a clear hysteresis loop at high relative pressures (P/Po > 0.7) indicative for the existence of
mesoporosity. The BJH/HK pore size distributions of the ZY@SiC suggest that they contain a
combination of micropores inherited from the ZY and meso/macro pores inherited from the SiC
that are in close proximity due to the higher dispersion of ZY particles on SiC surface (Fig. 6,
Table 1). Overall, it showed the co-existence of micro-mesoporosity in the ZY@SiC samples
constructed through the assembly of ZY and β-SiC.
3.3. Catalytic performance of SiC supported Zeolite-Y catalysts
The synthesized SiC supported ZY catalysts were evaluated for n-heptane cracking. In or
der to benchmark the catalytic performances of the synthesized ZY@SiC samples, the commerci
ally available zeolite-Y (CBV-600) and physically mixed catalyst of ZY/SiC-50 were also consid
ered for n-heptane cracking under the same testing conditions. All the catalysts were tested in n-h
eptane cracking at a temperature range of 425 to 475 °C. We studied n-heptane cracking at relativ
ely low temperature range in order to magnify the differences in catalytic performance as most of
reported zeolite based catalysts were studied over 500 °C [54-57]. The catalytic performances of
all ZY@SiC samples are presented in Fig. 7. It is apparent that the conversion of n-heptane and y
ields of cracking products increase with temperature which hints at a kinetically controlled or ma
ss transfer controlled process. Comparing the ZY@SiC catalysts under test, the highest conversio
n is attained by ZY@SiC-50 catalyst which is ~1.7 times higher in comparison to the reference C
BV-600 catalyst (Fig. 7a). Interestingly, all ZY@SiC catalysts exhibited higher reaction rates (ca
lculated on the basis of zeolite amount) when compared to CBV-600. Notably, the higher reactio
20

n rate is observed by ZY@SiC-50 catalyst which is ~3.5 times higher than CBV-600 catalyst (Fi
g. 7b).
(b)
(a)
(c)
(d)
Fig. 7. Conversion of n-heptane at different temperate in reaction time of 60 min over all studied
catalysts (a); Observed reaction rate for all catalysts calculated at different temperature (b) Yield
of C2-C4 products (c); Yield of C3 products obtained at different temperature over all studied
catalysts (d) (reaction time : 60 min, WHSV : 8 h-1).
Considering that the nature of acid sites in all the studied catalysts is expected to be similar. The
higher reaction rate displayed by ZY@SiC catalysts can be attributed to the enhancement in mass
transfer rates by the virtue of hierarchical ZY@SiC pores assembly and more accessibility to active
sites due to the uniform size zeolite crystals decorated on outer surface of SiC. The cracking prod
21

ucts distribution of all catalysts studied at different temperatures are presented in Fig. 7c. The yie
lds of C2-C4 products improve with increasing temperature. The cracking product yield and C3 pr
oduct formation also varied across the studied catalysts. The catalyst ZY@SiC-50 showed higher
yield of C2-C4 products in comparison to CBV-600 at similar reaction conditions (Fig. 7). The hig
hest C3 yield (up to 16.2%) was attained with the catalyst ZY@SiC-50 compared to CBV-600 (C
3 yield, 9.8%). The catalytic performance of ZY@SiC-50 was also compared with physically mix
ed catalyst (ZY/SiC-50). This comparison is expected to elucidate the extent of mass transfer enh
ancement due to the uniform dispersion of ZY particles on SiC surface and the creation of hierarc
hical assembly. The conversion and cracking products yield obtained with ZY/SiC-50 catalyst are
considerably lower than ZY@SiC-50 (Fig. 7). It is worth to mention that the catalytic performanc
e of ZY/SiC-50 was even lower than the commercial CBV-600 catalyst. This result highlights the
crucial role of hierarchical micro-mesopores assembly of ZY@SiC catalysts for enhancing the
diffusion of reactants/products and active sites accessibility.
With regard to light olefins yield, it can be seen that the catalysts (ZY@SiC-50 and ZY@
SiC-30) exhibit higher light olefins yields (ethylene, propylene and butylene) in addition to C3 pr
oduct yield (Table 2). The catalyst CBV-600 achieved only 12 to 14% light olefins yield and 6.4
to 8% yield towards propylene at 30 and 60 min respectively (Table 2, Fig. 8). Whereas, the ligh
t olefins and propylene yield considerably enhances up to 33% and 14.6% respectively for the ZY
@SiC-50 catalyst resembling a two-fold improvement in target product quality. Moreover, the ol
efin to alkane ratio of C3 (C3=/C3-) is 1.9 by CBV-600 which dramatically improves to 4 in the Z
Y@SiC-50 catalyst (Fig. 8). The aforementioned results with ZY@SiC catalysts clearly highlight
the influence of the hierarchical pores assembly of ZY@SiC not only in enhancing the conversio
ns but also beneficial in improving the desired product yield.
22

Table 2 Product distribution and light olefins yields for n-heptane cracking over CBV-600 and the
ZY@SiC catalystsa
Catalyst
Yield (%)
Time
(min) (%)
Conv.
Y Propene
(%)
Y light olefins
(%)e
b
c
d
C1
C2
C3
C4
CBV-600
36
38
2.9
2.7
6.1
2.7
1.5
2.2
2.5
5.4
2.1
1.3
5.0
6.5
10
12.2
13.7
18.5
7.5
13.7
13.3
17.9
7.7
8.0
9.5
14.6
5.8
4.0
6.4
7.8
13.0
3.8
2.7
14.0
17.5
33.0
12.8
6.4
ZY@SiC-30
ZY@SiC-50 30
ZY@SiC-70
ZY/SiC-50
56
25
22
28
31
49
19
16
5.5
2.9
4.5
5.9
8.8
4.2
2.1
7.7
7.0
CBV-600
9.8
9.5
12.0
15.2
29.9
9.3
ZY@SiC-30
ZY@SiC-50 60
ZY@SiC-70
ZY/SiC-50
10.9
16.2
4.9
9.9
15.7
6.5
5.5
5.1
5.3
a reaction temperature : 475 °C, WHSV : 8 h-1.
b C2 = ethane + ethylene, c C3 = propane + propene, d C4 = butane + butylenes
e light olefins (ethylene + propene + butylenes)
(a)
(b)
23

Fig. 8. Yield of light olefins (ethylene + propylene + butylenes) obtained at 475 °C over all studied
catalysts (a) Yield of propene (C3=), propane (C3-), propene/propane (C3=/C3-) ratio obtained at
475 °C, reaction time: 60 min, WHSV: 8 h-1 (b).
The influence of hierarchical structure on catalytic performance in cracking reaction has been rep
orted by several groups [54-57]. For instance, Jiao et al. reported a facile approach for the synthes
is of core-shell type hierarchically structure zeolite-Y@ γ-Al2O3 nanorods composites via control
addition of zeolite-Y crystals into boehmite nanorods aqueous sol [54]. The resulting zeolite-Y@
γ-Al2O3 composites exhibited enhanced textural properties and acid site accessibilities as a result,
improved the catalytic performance in the cracking of heavy oil fractions. Furthermore, the achie
ved gasoline yield was higher when compared to the sample prepared by physical mixing. The au
thors attributed the aforementioned improvements due to the homogeneous and high degree of di
spersion and the creation of hierarchical micro-mesoporosities via core-shell type arrangement be
tween the zeolite and the alumina matrix. Fan et al. prepared SiC foam coated with ZSM-5 based
catalyst by a dispersion based method aimed at testing the catalytic cracking of n-hexane [57]. The
uniformly deposited ZSM-5 on SiC foam exhibited significant improvement in catalytic
performance, higher C2-C4 olefins yield and anti-deactivation performance compared to as such
ZSM-5 catalyst. The authors concluded that the improvement in catalytic activity and light olefins
yield of ZSM-5 supported SiC foam catalyst is attributed to the removal of mass transfer limitation
and the effective contact surface between reactant/intermediates and the zeolite sites available on
surface of SiC foam support [57]. Our results corroborate the aforementioned studies.
In order to compare catalyst stability towards deactivation mechanisms, the catalytic crac
king of n-heptane was performed under similar condition for a total TOS of 150 min. Fig. 9a pres
ents the conversion of all catalysts as a function of time. The conversions of all catalysts were fou
nd to decrease with time which is a sign for catalytic deactivation. However, the ZY@SiC catalysts
24

are less prone to deactivation since the slope of the conversion with time curve is less steep. ZY@
SiC-50 catalyst showed best catalytic performance over the entire testing period. Most importantl
y, the stability of ZY@SiC-50 catalyst was much better in comparison to CBV-600 as well as ph
ysically mixes catalyst. The conversion of CBV-600 catalyst decreases significantly from 51 to 1
6% by the end of the testing period indicating rapid deactivation (69% conversion loss). The cata
lysts ZY@SiC-30, ZY@SiC-50, and ZY@SiC-70 experienced a conversion loss of 53%, 33%, a
nd 67% respectively. The deactivation constant (Kdec) for all the catalysts was calculated from the
kinetic plot of ln(Ct/Co) vs. time (Fig. 9b). The kinetic plots of all catalysts clearly displayed that
the deactivation of catalyst follow first order decay. The deactivation constant values (Kdec) for al
l catalyst are presented in Table S2. The catalyst CBV-600 and the physical mixed catalyst (ZY/
SiC-50) exhibited faster deactivation rate manifesting in higher deactivation constants, 7.5 x 10-3
min-1 and 8.3 x 10-3 min-1 respectively. On the contrary, ZY@SiC-50 exhibited the slowest deacti
vation rate as exemplified by its deactivation constant (2.8 x10-3 min-1) even when compared to C
BV-600 and physical mixed catalyst (ZY/SiC-50). On zeolites, deactivation occurs primarily by s
econdary reactions that result into the formation of coke. In the ZY@SiC, the faster diffusion rate
s reduce the time the products spend within the catalyst and as such decreases the chance for the o
ccurrence of secondary reactions.
The time evolution of the light olefins and propene yields for all the tested catalysts are pr
esented in Fig. 9c and Fig. 9d. The catalysts ZY@SiC-50 and ZY@SiC-30 exhibited higher light
olefins and propene yield in comparison to CBV-600. The highest light olefins yield ranging from
24 to 36% and propene yield (9.6-16%) was achieved by the catalyst ZY@SiC-50 followed by th
e catalyst ZY@SiC-30 which produced moderately higher light olefins and propene yield than C
BV-600.
25

(a)
(b)
(c)
(d)
Fig. 9. Conversion of n-heptane for all studied catalysts over total TOS of 150 min (a); Plots of ln
(Ct/Co) vs. reaction time for studied all catalyst (b); Yield of light olefins (c); Yield of propene (d)
obtained from all studied catalysts over total TOS of 150 min (reaction temperature : 475 °C,
WHSV : 8 h-1).
Overall, the ZY@SiC-50 catalyst displayed superior catalyst performance manifesting in higher
conversions, higher light olefin yields, and slower deactivation rates when compared to the other
catalysts. The archived results of n-heptane cracking with ZY@SiC catalysts clearly suggest that
the conversion and yield of desire products significantly influence by type of catalysts selected for
this study. In SiC supported zeolite catalysts, the optimum loading of zeolite-Y is necessary to
26

boost catalyst activity as well as to produce higher light olefins yield. The commercial zeolite
catalyst (CBV-600) in present study showed good conversion but less control over selectivity
which is possibly due to the large micro-size zeolite crystals. In the case of ZY@SiC catalysts, the
submicron crystal size of ZY crystals (apporx.400 nm) grown on surface of SiC and the assembling
of small size microporous zeolite crystals and macroporous SiC creates the hierarchical
organization of porous network. The homogeneously dispersion of uniform submicron size zeolite
crystals (acid sites) on SiC surface offers higher availability of active sites which impact on catalyst
activity. Whereas the hierarchical micro-mesopores ZY@SiC assembly effectively reduce diffuse
resistance for reactant and product molecules to/from active sites. As consequence, the light olefins
molecules easily diffuse out from pore channels without further cracking or secondary reactions
which overall beneficial in enhancement of desire product selectivity and catalyst life time. The
sample ZY@SiC-30 which possesses comparatively higher amount of independently grown ZY
crystals (due to the lower content of SiC) showed moderate improvement in catalyst activity and
yield than CBV-600. In the case of ZY@SiC-70, the zeolite content is too low which caused for
low catalyst performance. Therefore, the catalyst ZY@SiC-50 with better incorporation of zeolite
crystals on SiC surface outperforms other catalysts as well as commercial catalyst CBV-600. The
remarkable catalyst performance of ZY@SiC-50 catalyst in cracking reaction suggests that the
application of the present catalyst system can be extended to other catalytic application whereby
the advantages of SiC supported ZY assembly can be utilized.
4. Conclusions
The SiC supported zeolite-Y based catalysts were successfully synthesized by an in-situ
approach under hydrothermal condition using cationic template treated SiC. The cationic template,
PDDA due to its polymeric nature forms a uniform coating over SiC surface thus facilitating the
27

uniform growth of ZY on the SiC surface by acting as a linker. Furthermore, the quaternary
ammonium moieties of PDDA present on SiC surface maintain the interaction with zeolite seed
crystals thus directing the growth of ZY crystals around PD-SiC particles which led to the
formation of a hierarchical structure (ZY@SiC). Characterization of the ZY@SiC samples
revealed that the addition of PD-SiC in different amount did not disturb the formation of FAU
phase. Moreover, the resulting ZY@SiC assembly inherited the porosity of their underlining
components thus creating hierarchical micro-mesopores assembly caused by the uniform growth
of microporous ZY particles on β-SiC surface. In the catalytic cracking of n-heptane, the ZY@SiC
catalysts offer higher availability of active sites and allow fast diffusion of reactants and products
which played crucial role in improving catalyst performance, light olefin yield and catalyst lifetime
when compared to commercial zeolite-Y (CBV-600) as well as physically mixed catalyst. The
achieved results with ZY@SiC-50 clearly signify the importance of having hierarchical pores
assembly in enhancing reaction rate and desire product yield in cracking, which overall benefitted
to oil-refining industry.
Credit Author Statement
Aasif A. Dabbawala: Investigation, Methodology, Conceptualization, Formal analysis, Writing –
Original Draft
Balasubramanian V. Vaithilingam: Writing-Reviewing and Editing, Data Curation,
Visualization, Validation,
Hemant mittal: Formal analysis, Data Curation
Yasser Al Wahedi: Writing-Reviewing and Editing, Software, Supervision, Project
administration, Resources
Shairoz khan: Investigation, Data Curation
28