A mesoporous aluminosilicate prepared by simply coating fibrous γ-AlOOH on the external surface of SBA-15 for catalytic hydrocarbon cracking

Article abstract of DOI:10.1039/c6ra07467g

A binary SiO₂/Al₂O₃ composite with fibrous γ-alumina coating on the external surface of SBA-15 has been synthesized by simply mixing SBA-15 with fibrous boehmite sol, followed by aging and calcination. The textural and aci

Full text of DOI:10.1039/c6ra07467g

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A mesoporous aluminosilicate prepared by simply
coating fibrous g-AlOOH on the external surface of
SBA-15 for catalytic hydrocarbon cracking†
Cite this: RSC Adv., 2016, 6, 40296
*
*
Zongbo Shi, Yejun Guan, Peng Wu and Mingyuan He
A binary SiO2/Al2O3 composite with fibrous g-alumina coating on the external surface of SBA-15 has been
synthesized by simply mixing SBA-15 with fibrous boehmite sol, followed by aging and calcination. The
textural and acidic properties of the resultant material were characterized by various techniques, i.e.
powder X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared (IR) spectroscopy, N2
adsorption/desorption, nuclear magnetic resonance (NMR) spectroscopy, and temperature programmed
desorption of ammonia and ethanol (NH3-TPD and EtOH-TPD). Coating fibrous g-alumina stabilized the
SBA-15 structure against collapse under critical steaming conditions (800 ꢀC and 4 h) allowing for the
facile synthesis of mesoporous aluminosilicate. The migration of aluminum and silicon between the
alumina and SBA-15 zones led to the formation of Al–OH–Si with weak acidity which served as the
active sites with excellent catalytic performance in the cracking of 1,3,5-triisopropyl benzene (TIPB).
Received 22nd March 2016
Accepted 16th April 2016
DOI: 10.1039/c6ra07467g
www.rsc.org/advances
oligomerization, lube dewaxing and the conversion of methanol
to hydrocarbons.6,11,12 Ishihara and coworkers reported that
Introduction
hierarchical ZSM-5 containing mesoporous silica–aluminas
showed not only higher conversion of VGO but also higher
yields of gasoline compared with that of single ZSM-5.13 Zhuo
et al. prepared sprayed FCC catalyst by using a mixture of silica
sol and alumina sol as the binder and active component, which
exhibited improved heavy oil conversion and transport fuels
selectivity.14 The enhanced catalytic activity is likely related to
the mesopores and the transfer of aluminium and/or silicon
species from the matrix to the zeolite phase.15
Natural clays can be directly used as SiO2/Al2O3 source and
have been widely studied.16 Recently, it is found that coating g-
Al2O3 on the surface of SiO2 brings several benets: the crys-
talline alumina-containing aluminosilicate usually possesses
high hydrothermal stability in surface area, partly because the
g-Al2O3 was thermal stable in a range of temperature.17,18 Jin
et al. demonstrated that SiO2@Al2O3 powders possess higher
thermal conductivity than SiO2 powder, which helped to delay
catalyst from overheating. On the other hand the SiO2@Al2O3
powders were of low surface area and showed limited catalytic
activity.19,20 It is highly desirable to synthesize the active SiO2–
Al2O3 matrix with high surface area. Yu and coworkers synthe-
sized SiO2@Al2O3 with the surface area of 653 m2 gꢁ1 using
Fluidized catalytic cracking (FCC) aims to convert heavy frac-
tions in vacuum distillate oil to clean liquid fuels with small
molecular sizes.1 The major active components of FCC catalysts
are FAU- or MFI-type zeolites that exhibit strong acidity and
excellent hydrothermal stability. Due to an increasing demand
to process heavier residues (boiling point > 400 ꢀC, carbon
number > 20), it is highly desirable that these bulky molecules
could undergo pre-cracking on acid sites which locate on the
external surface provided by the matrix.2 In this regard, devel-
oping FCC catalysts with high accessibility and active matrices
such as nonzeolitic aluminosilicates, also called amorphous
silicon alumina (ASA) with moderate acidity and mesoporosity
are quite promising.3,4 Moreover, the matrices should be resis-
tant to the severe deactivation conditions of the practical
regeneration, with temperatures above 700 ꢀC or even higher in
steam.5–7
Recent years have witnessed substantial progress in
preparing active matrices such as Al2O3, SiO2 or binary SiO2/
Al2O3 composites with improved textural properties, enhanced
surface acidity, greater hydrothermal stability.8–10 Their prac-
tical applications used as a matrix have been widely explored in
many reactions such as hydrocarbon cracking, propylene
electrostatic attraction and
a heterogeneous nucleation
strategy, where the microspheres SiO2 with surface area of 999
m2 gꢁ1 were used as the core.21
Here we reported a facile synthesis of a binary SiO2/Al2O3
composite by coating brous g-alumina on the external surface
of SBA-15. This simple approach combined electrostatic
attraction strategy, aging and calcination, resulting in the
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, North
Zhongshan Rd. 3663, Shanghai 200062, China. E-mail: yjguan@chem.ecnu.edu.cn;
hemingyuan@126.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra07467g
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formation of weak acid sties by the migration of silicon and ground for 0.5 h in an agate mortar. Then the mixture was
ꢁ1
ꢀ
ꢀ
aluminum species inside the matrix. Thus obtained binary calcined at 550 C for 6 h with a heating rate of 2 C min
.
SiO2/Al2O3 composite can efficiently catalyze the cracking of To compare the hydrothermal stability of the aluminosili-
TIPB to diisopropyl benzene, cumene, benzene and propylene. cates, the steamed samples, namely steamed Al2O3, steamed
SBA–Al2O3 and steamed SBA/Al2O3-imp were obtained by
calcining Al2O3, SBA–Al2O3 and SBA/Al2O3-imp at 800 ꢀC in
100% steam for 4 h, respectively.
Experimental
Chemicals
Non-ionic triblock co-polymer Pluronic P123 (EO20PO70EO20,
Characterization methods
molecular weight ¼ 5800) were purchased from J&K Chemical
Powder X-ray diffraction (XRD) patterns were collected on
Ltd. HCl (37 wt%), AlCl3$6H2O, Al(NO3)3$9H2O, NaOH, tetrae-
a Rigaku-Ultima diffractometer with a nickel lter using a Cu
thylorthosilicate (TEOS), aluminum isopropoxide, hexane all in
AR grade, were supplied by Sinopharm Chemical Reagent Co.
Ltd. The chemicals were used directly without further
purication.
Ka radiation source (l ¼ 0.15418 nm, 35 kV, 25 mA) in the 2q
range from 0.5–5ꢀ (0.5ꢀ minꢁ1) and 10–80ꢀ (10ꢀ minꢁ1). Scan-
ning electron microscopy (SEM) was performed directly
without spraying gold on a Hitachi S-4800 microscope. Infrared
(IR) spectra were recorded on a Nicolet Fourier transform
Preparation of binary SiO2/Al2O3 composite
infrared spectrometer (NEXUS 670). The self-supported wafers
Binary SiO2/Al2O3 composite was synthesized by mixing SBA-15 without mixing with KBr (about 10 mg, B 1 cm) were pretreated
ꢀ
with brous boehmite sol, followed by aging and calcination. in the IR cell under vacuum at 350 C for 1 h and then the IR
First SBA-15 was synthesized according to the procedure re- spectra of OH group were recorded. Aer the samples were
ported by Zhao et al.22 Typically, 2.0 g Pluronic P123 was dis- cooled down to room temperature, pyridine vapor was dosed
solved in 60 mL of 2.0 M HCl and 15 mL of distilled water, to into the IR cell. The physically adsorbed pyridine was removed
ꢀ
which 4.3 g of TEOS was added. The resulting mixture was by evacuation at 150 C. N2 adsorption–desorption isotherms
ꢀ
continuously stirred at 40 ꢀC for 24 h, and then transferreꢀd into were measured at ꢁ196 C on a Quanta chrome Autosorb-3B
a Teon-lined stainless steel autoclave and aged at 100 C for volumetric adsorption analyzer. Before the measurements,
24 h. Aer crystallization, the solid product was ltered, washed the samples were degassed at 300 ꢀC for 6 h. The BET (Bru-
ꢀ
with deionized water repeatedly, and dried overnight at 100 C nauer–Emmett–Teller) specic surface area was calculated
in air. The as-synthesized sampꢁle1was calcined at 550 ꢀC for 6 h using adsorption data acquired at a relative pressure (P/P0)
with a heating rate of 2 ꢀC min
.
range of 0.05–0.30 and the total pore volume was determined
Then brous boehmite sol (AlOOH sol), brous boehmite from the amount adsorbed at P/P0 of about 0.99. The pore size
(AlOOH) and brous alumina (Al2O3) were synthesized accord- distribution (PSD) curves were calculated from the analysis of
ing to the procedure reported by Deng et al.23 Typically, 20 mmol adsorption isotherm branches using the Barrett–Joyner–
of AlCl3$6H2O was dissolved into 40 mL water under stirring at Halenda (BJH) algorithm. The bulk Si, Al and Na contents were
room temperature, to which 60 mL of 1 M NaOH was added determined by inductively coupled plasma emission spec-
dropwise. Then 1 M HCl was added to adjust the pH to ꢂ4. The trometry (ICP) on a Thermo IRIS Intrepid II XSP atomic emis-
obtained suspension was transferred into a Teon-lined stain- sion spectrometer. The nuclear magnetic resonance
less steel autoclave and aged at 175 C for 12 h. The resulting spectroscopy of 27Al (27Al NMR) were measured on a VARIAN
ꢀ
suspension was named as AlOOH sol. The AlOOH was collected VNMRS 400WB NMR spectrometer.
ꢀ
by centrifugation of AlOOH sol, and dried at 100 C overnight.
Temperature programmed desorption of ammonia (NH3-
The Al2O3 was obtained by calcination of AlOOH at 550 C for TPD). Aer pretreatment at 550 ꢀC under a 25 sccm owꢀ of
ꢀ
ꢁ1
ꢀ
6 h with a heating rate of 2 C min
.
helium for 2 h, each sample with 100 mg was cooled to 50 C,
SBA-15/brous boehmite composite (SBA–AlOOH) and SBA- and then adsorbed to saturation by ammonia for 30 minutes.
15/brous alumina composite (SBA–Al2O3) were obtained as Ammonia physically adsorbed on the catalyst was removed by
following: 20 mmol SBA-15 was added into the brous boehmite ushing the sample under a 25 sccm ow of helium for 2 h at
sol (containing 20 mmol Al2O3) under continuously stirred at the adsorption temperature. Thermal desorption of ammonia
room temperature for 2 h. The resulting suspension was was carried out in the tempeꢁra1ture range of 100–550 ꢀC
ꢀ
transferred into a Teon-lined stainless steel autoclave and increasing at a rate of 10 C min
aged at 150 C for 4 h, then cooled down naturally. The SBA–
.
ꢀ
Temperature programmed desorption of ethanol (EtOH-
ꢀ
AlOOH product was collected by ltration, washed with distilled TPD). 100 mg of sample was rst ushed in helium at 400 C
water and dried at 80 ꢀC overnight. Thꢀe SBA–Al2O3 was obtained with a 25 sccm ow of helium for 2 h. Aer the sample was
by calcination of SBA–AlOOH at 550 C for 6 h with a heating cooled down to 30 ꢀC, helium with saturated ethanol vapor was
ꢁ1
ꢀ
rate of 2 C min
.
passed through the sample for 10 min allowing for the
For comparison, alumina incorporated SBA-15 (SBA/Al2O3- adsorption of ethanol. Physically adsorbed ethanol was
imp) with the same SiO2/Al2O3 ratio was also prepared by removed by ushing under a 25 sccm ow of helium at 30 ꢀC for
impregnation according to the reported procedure.24 The 2 h. Thermal desorption was carried out in the temperature
ꢁ1
ꢀ
mixture of 10 mmol Al(NO3)3$9H2O and 10 mmol SBA-15 was range of 30–400 ꢀC at a ramp rate of 10 C min
.
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Catalytic cracking of triisopropyl benzene
A stainless steel tube micro reactor with an inner diameter of 10
mm was used for the cracking experiments. The samples were
all ammonium exchanged before catalytic test. 1.0 g of the
catalyst particles (40–60 mesh size) were loaded and in situ
ꢀ
activated at 400 C for 2 h under a 40 sccm ow of N2. Aer-
wards, N2 was switched off and 1.506 g TIPB was injected into
the carburetor with the ow rate of 1.29 g minꢁ1. TIPB was
immediately vaporized and owed into the reactor. The reaction
products were cooled with an ice-water bath. Aer the reaction,
the N2 was switched on for purging the residual products, with
a ow rate of 40 sccm for 10 min. The weight of liquid products
and the volume of gas products were recorded. The liquid
products were analyzed by an offline gas chromatograph (GC)
equipped with a ame ionization detector (FID) and a HP-Plot Q
column (B 0.32 mm and 50 m length). The gas products were
analyzed by an online GC equipped with a thermal conductivity
detector (TCD), a FID and a DM-alumina column (B 0.53 mm
and 50 m length). Almost no TIPB thermal cracking was
observed at 400 ꢀC without catalyst.
Fig. 1 Small-(left) and wide-(right) angle XRD patterns of the as
synthesized SBA-15 (a), Al2O3 (b), and SBA–Al2O3 (c).
The refractory property of the coke formed on each catalyst
was studied via TPO technique.25 In a typical experiment, 50 mg
sample was pretreated at 400 ꢀC under 25 sccm ow of helium
for 1 h and cooled to 100 ꢀC. Aerwards helium was switched off
and 40 sccm ow of 20 vol% O2/N2 was passed through the
coked catalyst at 100 ꢀC for 0.5 h. Then the sample was heated
from 100 C to 800 C with ramp rate of 10 C minꢁ1. The exit
stream was passed through a CuO/Cu2O catalyst bed held at 500
ꢀC which converted the CO formed and any desorbing hydro-
carbons to CO2 and H2O.26 Aer removing the water by passing
through a neutral desiccant, the CO2 was measured by a TCD
monitor and the amount of the coke was calibrated with
commercial carbon as reference.
ꢀ
ꢀ
ꢀ
Fig. 2 SEM images of the as-synthesized SBA-15 (a), AlOOH (b), SBA–
Al2O3 (c), and the schematic representation of SBA–Al2O3 synthesis
(d).
Results and discussion
Characterization of SBA–Al2O3 materials
Fig. 1 displays the X-ray diffraction patterns of the SBA-15, Al2O3
and SBA–Al2O3 samples. The SBA-15 sample illustrated three
well resolved peaks at 2q ¼ 1.02ꢀ, 1.74ꢀ, 2.00ꢀ (Fig. 1a, le),
corresponding to the (100), (110) and (200) diffractions of a 2D
hexagonal space group (P6mm), respectively. The unit cells
calculated by the XRD pattern are ꢂ10 nm. For SBA–Al2O3,
similar diffraction peaks due to SBA-15 were observed but with
lower intensity (Fig. 1c). New peaks assigned to g-alumina
(JCPDS card number 04-0880) were found.
Fig. 2 shows the typical SEM images of the resulting mate-
rials. SBA-15 particles (Fig. 2a) exhibit hexagonal cylinder like
morphologies, which is consistent with the XRD results. The
brous boehmite (AlOOH) particles (Fig. 2b) had uniform
brous-like morphology, with length in 0.5–1 mm and diameter
in 6–9 nm. Fig. 2c shows the morphology of SBA–Al2O3
composite. Aggregates of brous Al2O3 were clearly observed in
the image, while the SBA-15 particles were hardly seen. We
envisage that Al2O3 bers were homogeneously dispersed on the
external surface of SBA-15 during the preparation. The brous
structure of AlOOH likely retained in the subsequent calcina-
tion step. Cheng et al. reported that the Al2O3 colloid particles
could deposit on the surface of SiO2 microspheres via electro-
static attraction strategy at the pH value between the isoelectric
points of AlOOH and SiO2.21 It should be emphasized that the
pH value of the synthesis solution (pH ¼ 4–5) was appropriate in
preparing microspheric SiO2@Al2O3 composites since the
isoelectric points of AlOOH and SBA-15 are at pH values of 9.1
and 2.3, respectively.27,28
Fig. 3 displays the N2 adsorption/desorption isotherms and
pore size distribution curves of the as-synthesized SBA-15, Al2O3
and SBA–Al2O3 samples. As shown in Fig. 3a, the SBA-15 dis-
played a typical IV isotherm with two sharp inections in the
relative pressure range of 0.70–0.80 and an H1-type hysteresis
loop. The BET surface area and pore size of the parent SBA-15
sample are 906 m2 gꢁ1 and 7.5 nm (Table 1, 1), respectively. A
signicant quantity of microporous surface area of SBA-15
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Al2O3. The SBA/Al2O3-imp (Fig. 3c) showed one-step adsorption
isotherm and step-wise desorption isotherm. The step-wise
desorption isotherm is due to the fact that the encapsulated
mesopores empty at lower pressure than the open pores of
similar size.29 The BET surface area of the SBA/Al2O3-imp was
246 m2 gꢁ1
.
The hydrothermal stability of silica–alumina composites is
of vital importance in practical application. We therefore
hydrothermally treated the above-mentioned materials with
100% steam at 800 ꢀC for 17 h and then tested their textural
properties. The surface areas of steamed Al2O3, SBA–Al2O3 and
SBA/Al2O3-imp decreased to 100, 150 and 44 m2 gꢁ1, respec-
tively. This result suggests that the porous structure of SBA–
Al2O3 was more stable than SBA/Al2O3-imp in steam. Early study
proposed that Al2O3–SiO2 composites containing crystallized
alumina showed higher hydrothermal stability, which well
explained our nding that SBA–Al2O3 showed excellent hydro-
thermal stability. By contrast the surface area of SBA/Al2O3-imp
decreased sharply since amorphous Al2O3 was present (ESI,
Fig. S1†). The XRD patterns of the steamed catalysts were also
shown in Fig. S1.† The steamed brous alumina sample
transferred to d-alumina (JCPDS card number 23-1009), while
the crystalline structure of g-alumina phase in the SBA–Al2O3
was retained.
Fig. 3 N2 adsorption/desorption isotherms and pore size distribution
curves (inset) of the catalysts before (black, top) and after (blue,
bottom) steaming at 800 ꢀC for 4 h: SBA-15 (a), Al2O3 (b), SBA–Al2O3
(c), comparison sample SBA/Al2O3-imp (d).
To further characterize the evolution of silicon and
aluminum species in the synthesis, these samples were sub-
jected to 27Al MAS NMR analysis and Fig. 4 displays the spectra
of the as-synthesized samples. The NMR of AlOOH sample
(Fig. 4a) shows a single peak at ꢂ6 ppm, which attributes to the
octahedral coordinated aluminum in the well crystallized
boehmite.30 Aer calcination, the boehmite phase (AlOOH)
turned to Al2O3 phase. Accordingly, peak at ꢂ65 ppm was
noticed (Fig. 4b) assigned to tetrahedral coordinated aluminum
atoms in clusters of aluminum oxide. Fig. 4c displays the NMR
of SBA–AlOOH and two peaks at ꢂ54 and ꢂ6 ppm are found.
The latter one is due to the octahedral coordinated aluminum of
AlOOH, while the former one is attributed to the tetrahedral
coordinated aluminum in the vicinity of terminal silanols.16,31
These tetrahedral coordinated Al sites generally show weak
Bronsted acidity and have found wide applications in catal-
ysis.32 According to Fig. 4c and d, the ratio of tetrahedral Al
species in the vicinity of terminal silanols is estimated to be
sample was found (321 cm2 gꢁ1) which is attributed to the
microporous corona in the mesopores walls. Fig. 3b displays the
characteristic type IV isotherm for the Al2O3 sample, where the
adsorbed amount continuously increased with the increase of
P/P0, corresponding to the BET surface area of 225 m2 gꢁ1 and
the pore size of 9.8 nm (Table 1, 2). The SBA–Al2O3 sample
(Fig. 3c) displayed a typical IV isotherm, relevant to the BET
surface area of 425 m2 gꢁ1, total pore volume of 0.97 cm3 gꢁ1
and the pore size of 8.9 nm (Table 1, 3). The microporous
surface area of the SBA–Al2O3 sample was greatly diminished
compared with that of the SBA-15 sample.
We also investigated the porous structure of aluminated
SBA-15 prepared by conventional impregnation method (deno-
ted as SBA/Al2O3-imp). The Si/Al ratio of 1.04 was chosen for
SBA/Al2O3-imp, and the value was similar to that (1.12) of SBA–
Table 1 Physical properties of the as-synthesized samples
Area (m2 gꢁ1
)
Volume (cm3 gꢁ1
)
BJH diameter
(nm)
No.
Samples
Smicro
Sexternal
SBET
Vmicro
Vexternal
Vtotal
Fresh catalyst
1
2
3
4
SBA-15
Al2O3
SBA–Al2O3
SBA/Al2O3-imp
321
0
42
84
585
225
383
162
906
225
425
246
0.13
0.00
0.01
0.03
0.99
0.41
0.96
0.24
1.12
0.41
0.97
0.27
7.4
9.8
8.9
6.5
Steamed at 800 ꢀC for 4 h
5
6
7
Steamed Al2O3
Steamed SBA–Al2O3
Steamed SBA/Al2O3-imp
0
4
12
100
146
32
100
150
44
0.00
0.00
0.01
0.61
0.60
0.19
0.61
0.60
0.20
33.0
18.7
23.7
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bands due to Al–OH. The IR results suggest that the reaction
between surface Si–OH in SiO2 and Al–OH species in AlOOH
occurred in the preparation, likely leading to the presence of Si–
OH–Al sites. It should be noted that the acid strength of thus
formed Si–OH–Al species (3745 cmꢁ1) differs from that of
bridge OH species in zeolites.
The acidic properties of these materials were further studied
by both pyridine adsorption and NH3-TPD. Fig. 6 displays IR
spectra of pyridine absorbed on the catalysts aer desorption at
ꢁ1
ꢀ
150 C. The Al2O3 displayed a band at 1450 cm ascribed to
pyridine adsorbed on Lewis acid sites.35,36 Pyridine adsorbed on
SBA–Al2O3 sample showed two bands at 1544 and 1453 cmꢁ1
related to Bronsted sites and Lewis sites, respectively. The ratio
of Bronsted acid sites to Lewis acid sites (B/L) for SBA–Al2O3
sample was lower than that for SBA/Al2O3-imp sample. This
result is consistent with the NMR nding that limited amount
of tetrahedral coordinated aluminum in the vicinity of terminal
silanols were formed, as shown in Fig. 4d. Aer steaming, the
ratio of B/L for the SBA–Al2O3 remarkably increased and sur-
passed that of SBA/Al2O3-imp catalyst. The total amounts of
acid site on these samples were determined by NH3-TPD (see
Table 2 and Fig. S2†). The acid site density of SBA–Al2O3 was
0.53 m2 g ꢁ1, much higher than that of Al2O3 and SBA/Al2O3-imp
sample, in accordance with the higher surface area. Aer
steaming at 800 ꢀC for 4 h, the SBA–Al2O3 sample remained
26.4% of the total acid sites, but the SBA/Al2O3-imp sample only
remained 18.8% of the total acid sites.
Fig. 4 27Al MAS NMR spectra of the as synthesized AlOOH (a), Al2O3
(b), SBA–AlOOH (c), SBA–Al2O3 (d). The dash lines represent the
deconvoluted resonances.
about 2% of the total Al atoms, and it increases to about 5%
upon calcination.
EtOH-TPD has been shown to be a probe reaction in char-
acterizing the reactivity of surface acid sites (OH groups) on
aluminum based oxides by monitoring desorption of ethanol
and ethylene. SBA-15 was almost inert in ethanol dehydration to
ethylene at temperature below 200 ꢀC (Fig. 7a).37,38 The Al2O3
sample (Fig. 7b) exhibited two desorption peaks at ꢂ85 and
ꢂ260 ꢀC. The former one corresponds to desorption of medium-
strongly bonding ethanol, and the latter one is ascribed to the
desorption of formed ethylene on the Al–OH of alumina.38–41
The ethylene desorption peak on SBA–Al2O3 shis to higher
Fig. 5 displays the OH stretching vibrations between 3200
cmꢁ1 and 3800 cmꢁ1 for the SBA-15, Al2O3 and SBA–Al2O3
samples. As shown in Fig. 5a, the SBA-15 showed two bands at
ca. 3745 and 3679 cmꢁ1, both corresponding to the stretching
mode of Si–OH groups which are free of hydrogen bonding.33
Three characteristic absorption bands of OH groups of Al2O3
were observed at 3720, 3671, and 3586 cmꢁ1 (Fig. 5b).34 Fig. 5c
shows the OH vibration of SBA–Al2O3 sample and only bands at
3745 and 3679 cmꢁ1 assigned to Si–OH were noticed, and those
peaks corresponded to alumina were hardly seen. For compar-
ison, the IR spectroscopy of the physical mixture of SBA-15 and
Al2O3 is also present (Fig. 5d), which clearly shows the vibration
Fig. 6 IR spectra of pyridine absorbed on Al2O3, SBA–Al2O3, and
reference SBA/Al2O3-imp before (solid) and after (dash) steaming at
800 ꢀC for 4 h (B/L represents the molar ratio of Bronsted acid sites to
Fig. 5 IR spectra (OH region) of the SBA-15 (a), Al2O3 (b), SBA–Al2O3 Lewis acid sites. The absorption coefficients for Bronsted acid sites and
(c), and physical mixture of 43 wt% Al2O3 and 57 wt% SBA-15 (d).
Lewis acid sites are 3.03 and 3.80, respectively).
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Table 2 Comparison of acidity and TIPB cracking activities of various catalysts and their steamed counterparts
Product yield,b wt%
No.
Catalyst name
Acid sitesa (mmol gꢁ1
)
Conv.b (%)
Propylene
Benzene
Cumene
DIPB
Other CH
Coke
Fresh catalyst
1
2
3
Al2O3
SBA–Al2O3
SBA/Al2O3-imp
0.48
0.53
0.32
2.7
57.7
23.7
0.5
12.5
4.3
0.0
0.8
0.6
0.1
6.6
2.6
1.9
32.4
12.8
0.1
3.2
1.5
0.1
2.2
1.9
Steamed at 800 ꢀC for 4 h
4
5
6
Steamed Al2O3
Steamed SBA–Al2O3
Steamed SBA/Al2O3-imp
0.11
0.14
0.06
0.3
31.5
8.7
0.1
6.6
1.6
0.0
0.4
0.0
0.0
1.9
0.3
0.2
20.0
6.1
0.0
1.4
0.3
0.0
1.2
0.4
a
b
Determined by NH3-TPD measurement. Conv. of TiPB is dened as the sum of propylene, benzene, cumene, diisopropylbenzene (DIPB) and
other hydrocarbons. DIPB is the sum of 1,2-DIPB, 1,3-DIPB and 1,4-DIPB. The total carbon recovery was >95%.
coated on SBA-15. Second, newly formed tetrahedral coordi-
nated aluminum (2%) in the SBA–AlOOH sample was found and
its content further increased to 5% upon calcination according
to 27Al-NMR. This recombination of Si and Al atoms undoubt-
edly leads to the formation of weak Bronsted acid sites as well as
increased acid density according to pyridine adsorption and
NH3-TPD. Last, the higher ethylene desorption peak observed in
ethanol desorption tests also pointed to the formation of
different acid sites on SBA–Al2O3 from that of pristine Al2O3
phase. Caillot et al. found that desorption peak of ethylene
shied to higher temperature when silica was graed on the Al–
OH of alumina surface due to the loss of Lewis Al3+ sites while
forming Bronsted acid sites.38 These results conrm that weak
Bronsted sites preferably formed by transfer of Si species from
the SBA-15 to the AlOOH or Al2O3 surface, therefore resulting in
an acidic binary SBA–Al2O3 material with high surface area and
large mesopore.
Fig. 7 EtOH-TPD profiles of SBA-15 (a), Al2O3 (b), and SBA–Al2O3 (c).
temperature of ꢂ280 ꢀC as compared to that on Al2O3, as
demonstrated in Fig. 7c.
The main target of this study is to synthesize a binary SiO2–
Al2O3 composite with large surface area, improved hydro-
thermal stability and suitable acidity in a simple way. To this
end, high surface area SBA-15 and brous AlOOH materials
were chosen as the Si and Al precursors, respectively. The
resulted SBA–Al2O3 showed surface area of 425 m2 gꢁ1 and total
pore volume of 0.97 cm3 gꢁ1, which are both larger than
commonly seen binary oxides. SEM results suggest that the
morphologies of SBA-15 and AlOOH did not change during the
synthesis, while boehmite phase AlOOH turned to crystalline g-
Al2O3, which is thought to stabilize the SBA-15 structure from
collapse upon calcination. Moreover, the brous morphology
may also contribute to the hydrothermal stability of the resulted
material based on the nding that least loss of surface area was
observed for SBA–Al2O3 aer steaming at 800 ꢀC. Another
important issue of binary Si–Al oxide synthesis is the intro-
duction of acid sites, which was also achieved by this simply
method. It is well known that migration of silicon and
aluminum species into each other leads to the formation of
weak Si–OH–Al sites.15,42 This process is unambiguously
observed by the above mentioned characterization techniques.
First the OH vibration due to Al–OH disappeared aer being
Catalytic performance
Cracking is the process whereby bulk organic molecules are
broken down into smaller molecules such as light hydrocar-
bons. Zeolite, with high microspore surface area and high
acidity, serves as the main active component of FCC catalyst.
But some of the bulky hydrocarbons in heavy residues are larger
in size than the opening of the microspores of zeolite, which
need pre-cracking on the external surface of the zeolite and/or
matrix.2 To explore the potential application of thus prepared
SBA–Al2O3 as a matrix in FCC, cracking of TIPB with kinetic
˚
diameter of 9.4 A was carried out.
Table 2 lists the activities of TIPB cracking on various
materials. Al2O3 exhibited a TIPB conversion of 2.7% (Tables
2, 1). By contrast, the SBA–Al2O3 sample gave the TIPB conver-
sion of 57.7% (Tables 2, 2). As expected, propylene, benzene,
cumene, and diisopropyl benzene (DIPB) were the most prom-
inent products. The by-products included light hydrocarbons,
aromatics and cokes.38,43 The comparison SBA/Al2O3-imp
sample exhibited the TIPB conversion of 23.7% (Table 2, 2–3).
Generally, the cracking activity are controlled by the acidity and
accessibility of the catalysts.44–46 Only weak Lewis acid sites are
present on Al2O3 therefore very low activity was observed for
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Al2O3.38 SBA–Al2O3 catalyst showed higher TIPB conversion than
the SBA/Al2O3-imp, because of its higher amount of total acid
sites and more open mesoporous channel. Early studies have
also shown that introducing mesopores into the zeolites, i.e.
ZSM-5, MCM-22, remarkably enhances their catalytic activities
in TIPB cracking.47,48 It is believed that the effective diffusion
length of bulky molecules in large pores or hierarchical zeolites
can be obviously shortened, thus allowing for the improved
utilization of acid sites.49 On the other hand, this structural
property also led to considerably higher coke yield for SBA–
Al2O3 (Fig. S3†). Aer steaming at 800 ꢀC for 4 h, the initial
catalytic activities of SBA–Al2O3 and SBA/Al2O3-imp were 31.5%
and 8.7%, respectively (Table 2, 5–6). The loss in TIPB conver-
sion is in line with the decrease of surface area aer steaming in
critical conditions. Compared with SBA/Al2O3-imp, steamed
SBA–Al2O3 still showed much higher reactivity for the cracking
of TIPB thanks to its strengthened acidity by the steaming
treatment. The activity difference of SBA–Al2O3 and SBA/Al2O3-
imp aer steaming again points to the vital role of hydrother-
mally stable porous structure and acid sites in cracking.
Previous study also found that Al containing hexagonally
ordered MCM-41 showed high activity in TIPB cracking,
whereas completely lose its activity aer steaming at 600 ꢀC for
2 h. By contrast, a hydrothermally stable mesoporous alumi-
nosilicate still showed very strong acidity and high activity in
TIPB cracking.46
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Conclusions
In this work, the binary SiO2/Al2O3 composite was synthesized
by coating brous g-alumina on the external surface of SBA-15.
The SiO2/Al2O3 composite had the surface area of 425 m2 gꢁ1
and pore volume of 0.97 m3 gꢁ1, and no obvious blocked mes-
oporous was found. The crystalline g-alumina phase in the
mixture was proposed to stabilize the mesoporous structure of
SBA-15. The un-calcined binary SiO2/Al2O3 composite contained
2% of tetrahedrally coordinated aluminum which increased to
5% aer calcination. The binary SBA–Al2O3 composite prepared
by this facile means displayed much higher activity than the
SBA/Al2O3-imp material prepared by conventional impregna-
tion method in the cracking of bulky TIPB, and the catalytic
activity was well retained aer 100% steam treatment at 800 ꢀC
for 4 h. It was found that both Bronsted acid sites and Lewis
acid sites were present in the binary SBA–Al2O3 composite
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