Synthesis of a ZSM-5(core)/SAPO-5(shell) composite and its application in FCC

Abstract

A core/shell structure composite was synthesized via a new method of pre-coating one raw material. The composite was characterized by X-ray diffraction, SEM, TEM and N2 isothermal adsorption–desorption and Py-FTIR. In addition, the catalytic performance of the composite in cracking of heavy oil for producing olefin was also investigated. The characterization results show that the composite with a core/shell structure had smaller particle size, uniform SAPO-5 shell, and fewer acid sites than ZSM-5, accelerating the transport of reactant and product molecules between different zeolites. Consequently, the light olefins on the composites had high specific selectivity.

Full text

Synthesis of a ZSM-5(core)/SAPO-5(shell) composite
and its application in FCC
Qiang Zhang •Chunyi Li •Shaojun Xu •
Honghong Shan •Chaohe Yang
Published online: 13 April 2012
ÓSpringer Science+Business Media, LLC 2012
Abstract A core/shell structure composite was synthe-
sized via a new method of pre-coating one raw material.
The composite was characterized by X-ray diffraction,
SEM, TEM and N
2
isothermal adsorption–desorption and
Py-FTIR. In addition, the catalytic performance of the
composite in cracking of heavy oil for producing olefin was
also investigated. The characterization results show that the
composite with a core/shell structure had smaller particle
size, uniform SAPO-5 shell, and fewer acid sites than
ZSM-5, accelerating the transport of reactant and product
molecules between different zeolites. Consequently, the
light olefins on the composites had high specific selectivity.
Keywords Composite Microporous materials Core/
Shell Catalytic properties
1 Introduction
Most catalysts and additives for propylene synthesis com-
prise zeolite Socony Mobil #5 (ZSM-5) with a diameter of
0.54 nm and two types of interconnected 10-membered
ring channels. However, the relatively narrow pores of
ZSM-5 restrict its application in the heavy oil catalytic
cracking unit [1]. Consequently, numerous efforts have
exerted to overcome this limitation [2]. One of the most
effective approaches is the creation of micro-mesoporous
and micro-microporous composites. Despite many reports
concerning micro-mesoporous composites such as TUD-C,
TUD-M [3–5], and ZSM-5/MCM-41 [6–8], these materials
can not adapt to fluid catalytic cracking (FCC) unit steam
conditions at high temperatures. Thus, the fabrication of
micro-mesoporous materials with a high hydrothermal
stability and adjustable acidity remains to be one of the
greatest challenges in materials science.
Core/shell structure micro-microporous zeolite compos-
ites have relatively high hydrothermal stability. In contrast to
ZSM-5 zeolite, SAPO-5 shows larger pore and mild acidity,
though their acid strength is substantially higher than that of
the corresponding AlPO
4
materials. SAPO-5 showed good
the selectivity of light olefins when tested for its activity in a
catalytic conversion of heavy oil catalytic cracking unit.
ZSM-5(core)/SAPO-5(shell) composite would be possible
with competitive advantage because the shell zeolite does
not only confer mild acidity and larger pore to the inner
zeolite, but also shortened the diffusion distance of inter-
mediate products. Various types of zeolite composites can be
obtained by the isomorphic substitution, epitaxial growth or
overgrowth according to structure characteristic of the core
and shell zeolites. The composites can be obtained by the
isomorphic substitution of framework atoms when the core
and shell have similar frameworks, such as ZSM-5/silicalite.
This method can be easily implemented, but it is fit for few
zeolites. If the core and shell zeolites are composed of
identical building units but with different spatial arrange-
ments, the composites can be synthesized by epitaxial
growth, such as FAU/EMT [9,10], MFI/MEL [11], and
MOR/MFI [12–14]. The disadvantage of this type of mate-
rial is that some specific crystal faces must be assured. For
instance, the large single crystals MOR (core) crystals in
MOR/MFI composite adsorb nanoseeds via polycation
agents, and then take secondary crystallization. The
Q. Zhang (&)C. Li (&)S. Xu H. Shan C. Yang
State Key Laboratory of Heavy Oil Processing, China University
of Petroleum (East China), Qingdao 266555, Shandong,
People’s Republic of China
e-mail: girlzhangqiang@163.com
C. Li
e-mail: chyli@upc.edu.cn
123
J Porous Mater (2013) 20:171–176
DOI 10.1007/s10934-012-9586-x

preparation of the single crystals and nanoseeds in this step is
a very complicated process. The synthesis of the composites
with an entirely different zeolite structure can be achieved
using overgrowth, such as FAU/MAZ [15,16], BEA/MFI
[17], ZSM-5/SAPO-11 [18], SAPO-11/b[19], and MFI/AFI
[20–22]. The synthesis of combined aluminosilicate and al-
uminophosphate zeolite composites is more difficult because
the synthesis systems of the two zeolites are completely
different. ZSM-5(core)/SAPO-5(shell) composites with a
core/shell structure was synthesized by a two-step hydro-
thermal crystallization or embedding method [23]. However,
the composites contain a considerable amount of indepen-
dent shell zeolites and aggregated particles larger than
10 lm. The modified hydrothermal crystallization, embed-
ding method and vapor-phase transport were attempted, but
only independent shell zeolites were reduced slightly
[24–26].
Hence, a convenient synthesis strategy to combine
ZSM-5 and SAPO-5 into one core/shell composite is
desirable to solve the problems of aggregated particles and
independent shell zeolites. In the present study, an
improved synthesis procedure was developed to prepare
ZSM-5/SAPO-5 core/shell structure zeolite composites by
pre-coated material method. The overgrowth of a contin-
uous SAPO-5 polycrystalline shell around ZSM-5 was
successfully achieved. The composites ensure a higher
propylene yield and conversion of heavy oil.
2 Experimental
2.1 Composite zeolite and mechanical mixture
preparation
About 20 g of Nankai ZSM-5 was added to a solution of
8 g of phosphoric acid (85 %, Jinan Chemical Co.) in 30 g
of water. The mixture was treated under vigorous stirring at
363 K for 8 h, and dried at 457 K for 36 h. Then, 10 g of
ZSM-5 coated phosphorus was slowly added to 15 g of gel
with pseudo-boehmite (68 % Al
2
O
3
; industrial reagent
grade), silica sol (40 % SiO
2
; Qingdao Haiyang Chemical
Co., Ltd), and triethylamine (analytical reagent grade;
Sinopharm Chemical Reagent Co. Ltd., China) under vig-
orous stirring. The obtained mixture was placed in stainless
steel PTFE-lined autoclaves (50 ml), and hydrothermally
treated under static conditions for 24 h at 453 K. After
separation by filtration and washing with water, the resul-
tant product was dried at 393 K for 24 h in an oven and
calcined in a muffle furnace at 823 K for 4 h. About 20 g
of Nankai ZSM-5 and 10 g SAPO-5 were mixed together
under stirring to prepare the mechanical mixture. The ratio
of ZSM-5 and SAPO-5 is defined according to the methods
in literatures [27].
2.2 Catalyst preparation
The catalyst consisted of the following (by weight): 35 %
zeolite as the active part, 50 % kaolin as the matrix, and
15 % pseudo-boehmite gel as the binder. After complete
stirring, the slurry was dried, calcined at 813 K for 4 h, and
was treated for 4 h in a 100 % steam flow at 1,073 K. The
zeolite is ZSM-5, ZSM-5/SAPO-5, or composite zeolite.
2.3 Characterization
The phase identification of the samples was performed by
an X-ray diffraction (XRD) system (X’Pert MPD, Holland)
equipped with Cu Karadiation with an accelerating voltage
of 40 kV and a current of 40 mA. The crystal size and
morphology of the samples were determined by a Philips
FEI Quanta 200 scanning electron microscopy (SEM)
system. The elemental compositions were characterized by
an AXS Quantax 400 energy dispersive X-ray spectrometry
(EDS) system. Transmission electron microscopy (TEM)
images were obtained using a JEOL JEM-2010 micro-
scope. Brunauer–Emmet–Teller (BET) surface areas were
measured using an ASAP 2010 instrument (Micromeritics
Instrument Corp., USA).
In the Py-FTIR experiments, samples were dehydrated
at 773 K for 5 h, followed by adsorption of purified pyri-
dine vapor at room temperature for 2 h. Before measuring
the spectra, the sample in situ cell was heated at 473 K for
2 h in the N
2
flow to remove physically adsorbed pyridine.
2.4 Catalytic activity evaluation
The catalytic cracking performances were assessed using a
laboratory-scale fixed-bed microreactor unit with Daqing
vacuum gas oil as the feedstock. The reaction temperature
was 813 K, and the catalyst-to-oil ratio was 5 (wt/wt). The
cracked gas components were analyzed using a Varian GC
3800. The liquid was provided by an Agilent 6890 N GC
simulated distillation analyzer. The weight percentage of
coke in the catalyst was measured using a coke analyzer.
3 Results and discussion
3.1 XRD
The XRD data of ZSM-5, pretreated ZSM-5 and ZSM-5/
SAPO-5 are listed in Table 1. The final sample reveals the
characteristic peaks of the crystal phase of SAPO-5 besides
the diffraction peak of ZSM-5. This finding indicates that
the SAPO-5 phase has successfully formed and the crystal
phase of ZSM-5 has not changed during the secondary
crystallization process. The most diffraction peaks of the
172 J Porous Mater (2013) 20:171–176
123

P-ZSM-5 sample make a shift to high angle and a signifi-
cantly decreases comparing of this ZSM-5 because of
framework dealumination. The relative crystalline degree
of ZSM-5 decreased to 56 % from 100 %. The P-ZSM-5
and ZSM-5/SAPO-5 had the similar relative crystallinity of
ZSM-5. But the intensities of the ZSM-5 diffraction peaks
in the final sample are higher than in the pretreated ZSM-5
at 2h=7.95
o
, 8.81
o
and 8.90
o
. This suggests the (101),
(200) and (020) faces of ZSM-5 had been fixed during
crystallization. The relative crystalline degree of SAPO-5
in composite and mechanical mixture is 20 and 28 %,
respectively. This is because the incomplete crystallization
and smaller particle size of SAPO-5 during the secondary
crystallization process.
3.2 BET
Table 2shows that the Pore structure parameters of the
different samples. The specific surface areas (A
BET
)of
ZSM-5, SAPO-5, mechanical mixture and composite are
355, 232, 332 and 278 m
2
/g, respectively. The specific
surface area of ZSM-5 is the biggest, and that of SAPO-5 is
the smallest. The composite shows a smaller specific sur-
face area and pore volume than the mechanical mixture,
but the mesopore surface area and mesopore pore volume
are higher. These results, which are similar to those
observed in the synthesis of SAPO-11/Hbcomposites [19],
can be attributed to the low-surface- area SAPO-5 coating
on ZSM-5.
3.3 SEM and TEM
Figure 1shows the SEM images of composites. Prior work
indicated that the particle size of ZSM-5 is uniform (about
4lm), and the surface of the regular hexagonal particles is
smooth. Many small cracks were on the surface of pre-
treated ZSM-5 with because of the interaction between
ZSM-5 and phosphoric acid [27]. However, the cracks on
the pretreated ZSM-5 surface in the composite could not be
seen because they vanished or have been coated with shell
zeolites. Figure 1a shows that the shell crystals are uni-
formly and densely coated on the surface of ZSM-5. The
morphology of the composites is similar to those of ZSM-5
crystals, with the sizes ranging from 4 to 6 lm and the
shell thickness layers averaged 0.16 lm. This crystal size
is less than the average size (10 lm) of previously reported
ZSM-5/AlPO
4
-5 composites [22].
To confirm further the presence of SAPO-5 zeolites on
the surface of ZSM-5 in the composite, EDS analysis was
conducted. The results are shown in Table 3. Compared
with ZSM-5, phosphorus is observed in the composite. The
content of aluminum increases and that of silicon decrea-
ses. Figure 1b indicates phosphorus as a red dot homoge-
neously dispersed in the composite. Therefore, these thin
Table 1 The XRD data of the samples
ZSM-5 P-ZSM-5 ZSM-5/SAPO-5
2h(°) Intensity (cps) 2h(°) Intensity (cps) 2h(°) Intensity (cps) 2h(°) Intensity (cps)
7.96 7,260 7.96 1,545 7.95 3,299 7.42 10,125
8.81 4,214 8.84 951 8.81 2,010 14.86 3,127
8.91 3,464 8.92 878 8.90 1,735 19.68 9,859
23.07 17,591 23.13 11,458 23.05 9,683 21.10 7,295
23.28 14,091 23.32 9,060 23.26 8,592 22.37 16,801
23.70 6,285 23.73 4,355 23.67 3,891
23.91 8,402 23.91 5,806 23.88 4,742
24.39 6,247 24.37 4,076 24.36 3,621
The relative crystallinity 100 (ZSM-5) 56 (ZSM-5) 56 (ZSM-5) 20 (SAPO-5)
Table 2 Pore structure parameters of the different samples
Sample Surface area (m
2
/g) Volume (cm
3
/g) Pore diameter
(nm)
Micropore Mesopore Total Micropore Mesopore Total
ZSM-5 271 84 355 0.14 0.04 0.18 0.52
SAPO-5 235 36 271 0.10 0.08 0.18 0.70
Mechanical mixture 277 55 332 0.14 0.04 0.18 0.53
ZSM-5/SAPO-5 205 73 278 0.09 0.08 0.17 0.54
J Porous Mater (2013) 20:171–176 173
123

layers on the ZSM-5 surface are SAPO-5 crystals, and
these materials are indeed core/shell binary structure
composite zeolites.
To understand further the formation process of the core/
shell structure, the crystallization process is pursued at
different crystallization stages by TEM. When crystallized
for 12 h, these small particles are observed surrounding the
ZSM-5 crystals (Fig. 2a). Very small nanoparticles with
diameters of circa 10 nm could be observed at this stage
(Fig. 2b). These particles are the precursors of growing
SAPO-5 crystals. When crystallized for 24 h, the ZSM-5
cores are completely wrapped by a thin polycrystalline
SAPO-5 shell with an irregular morphology (Fig. 2c, d).
According to characterization results above, it is was
carried out that the composites with smaller size and even
thin SAPO-5 shell layer can be obtained by pre-coating one
raw material for synthesis of SAPO-5 on ZSM-5 and sec-
ond crystallization. The main reason is the phosphorus
as raw material for synthesis of SAPO-5 combines with
500 nm
(a )
3µm
(b)
Fig. 1 SEM images of the
ZSM-5/SAPO-5composite
samples
Table 3 Result of EDS analysis of the samples
Sample Mass fraction (%) SiO
2
/Al
2
O
3
(molar
ratio)
P
2
O
5
/Al
2
O
3
(molar
ratio)
Si Al P O
ZSM-5 40.73 1.20 0 58.07 65.11
ZSM-5/
SAPO-5
27.56 2.41 14.2 55.82 21.96 5.13
(a) (b)
b
(d)
SAPO-5
ZSM-5
200 nm
(c)
ZSM-5
SAPO-5
Fig. 2 TEM images of the
obtained composite under
different crystallization time:
aand b12 h, as well as (c) and
(d)24h
174 J Porous Mater (2013) 20:171–176
123

ZSM-5 by chemical bond to the extent that it is fixed to the
ZSM-5 and cannot move. After being added, template
agent and aluminum move to phosphorus and react with it
to form SAPO-5 crystals at ZSM-5 outface. This is similar
to synthesis of MCM-41/FAU composite by pre-adsorption
template agent [28]. Another reason is surface asperities
are caused by combination of P with ZSM-5 facilitate
adsorption of aluminum ion. Phosphorus is uniformly dis-
tributed and fixed at ZSM-5 surface before it reacts with
aluminum and template, so the SAPO-5 particle does not
stack and even distribute at ZSM-5 surface.
3.4 Acidity
IR spectroscopy of adsorbed pyridine is a technique for
measuring and distinguishing different types of acid sites
on zeolite. The IR spectra of pyridine adsorbed on different
zeolites are presented in Fig. 3. The two bands at 1,450 and
1,540 cm
-1
are related to the adsorption of pyridine mol-
ecules on Bro
¨nsted and Lewis acid sites, respectively
[29–31]. The IR spectra indicate ZSM-5 has the maximum
peak area at above two acid sites among all samples and
mechanical mixture is similar to ZSM-5. It can be observed
that a decrease in the Bro
¨nsted and Lewis acid sites occurs
for ZSM-5/SAPO-5 compared with ZSM-5, because
SAPO-5 with weak acid covers a large proportion of the
acid sites at ZSM-5 external surface.
3.5 Catalytic properties
The differences between the catalytic cracking perfor-
mances of the various catalysts are listed in Table 4. The
propylene yield using the ZSM-5-derived catalyst reaches
17.87 %. However, the conversion of heavy oil is only
76.78 %. The conversion and liquid petroleum gas (LPG)
yield increases by 0.7 and 2.6 % using the ZSM-5 and
SAPO-5 mechanical mixture-derived catalyst, respectively,
whereas the diesel yield decreases by 1.6 %. This is
because pore size of SAPO-5 is larger than that of ZSM-5;
larger molecules can be adsorbed in micropores of SAPO-5
to undergo shape-selective catalytic cracking reactions. But
pore size of SAPO-5 is 0.73 nm, and too large molecules
also can’t enter the pore of molecular sieves. The ZSM-5/
SAPO-5-derived catalyst produces the highest yield of
olefin and conversion of heavy oil, as well as the lowest
yield of diesel. This result can be attributed, at least in part,
to the presence of the shorter-channel and weak acidity
SAPO-5 and the coupling between ZSM-5 and SAPO-5.
The reactions of hydrocarbons are largely influenced by the
length of time hydrocarbon molecules spend inside zeolitic
micropores. The structure properties enhance the accessi-
bility to active sites [32,33]. The reaction of hydrocarbon
is largely influenced by the time that hydrocarbon mole-
cules spend inside the zeolitic micropores [34]. Given that
the composites comprise mesopores and a very thin SAPO-
5 shell layer, only a short time is needed for the interme-
diate product molecules to diffuse and further react.
Consequently, the LPG yield is higher. This finding is per-
fectly consistent with the concept of pore continuum
[35,36].
The higher yields of light olefins such as ethylene,
propylene and butylenes, benefit from thin SAPO-5 layer,
because its weak acidity and short channel stop the sec-
ondary reaction of terminal products in diffusing outward
the zeolite. In fact, the matching relation between weak
acidity and strong acidity and between 1-D and 3-D
channels often plays a critical role in producing light
olefins.
1400 1420 1440 1460 1480 1500 1520 1540 1560
Absorbance (a.u.)
Wavenumbers,cm-1
(a)
(b)
(c)
Fig. 3 IR Spectra of pyridine adsorbed aZSM-5, bMechanical
mixture, cZSM-5/SAPO-5
Table 4 Catalytic cracking results of the catalysts
Catalyst Yield (wt %) Conversion (%)
Ethene Propene Butene LPG Gasoline Diesel Coke
ZSM-5 3.64 17.87 13.74 38.17 18.19 10.22 4.62 76.78
Mechanical mixture 4.21 18.06 13.44 39.70 18.01 8.67 4.47 77.44
ZSM-5/SAPO-5 4.48 19.40 15.12 41.73 17.70 8.28 4.96 80.03
J Porous Mater (2013) 20:171–176 175
123

4 Conclusion
A ZSM-5/SAPO-5 core-shell structure composite is suc-
cessfully synthesized by a phosphorus-precoating method.
The composites exhibit a regular hexagonal platy shape
that resembles the original shape of ZSM-5 crystals, and
are about 4 lm in length. The very thin layer of the SAPO-
5 shell is coated on ZSM-5 to a moderate external surface
acidity. The unique pore structure and shorter shell thick-
ness of the composites facilitate mass transfer, and mark-
edly decreases the number of surface active sites. As a
result, the composite-derived catalysts exhibit excellent
performance in heavy crude oil cracking and olefin yield.
Acknowledgments The authors gratefully acknowledge the finan-
cial support of the Natural Scientific Foundation of Shandong Prov-
ince (ZR2009BQ018) and the Fundamental Research Funds for the
Central Universities.
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