The surface properties of aluminated meso–macroporous silica and its catalytic performance as hydrodesulfurization catalyst support
The surface properties of aluminated meso-macroporous silica and its catalytic performance as hydrodesulfurization catalyst support
Zhi-Gang Wang 0 1
Jia-Ning Pei 0 1
Sheng-Li Chen 0 1
Zheng Zhou 0 1
Gui-Mei Yuan 0 1
Zhi-Qing Wang 0 1
Guo-Qiang Ren 0 1
Hong-Jun Jiang 0 1
0 Shanghai Petrochemical Company of Sinopec , Jinshan, Shanghai 200540 , People's Republic of China
1 State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum , Beijing 102249 , People's Republic of China
Aluminated mesoporous silica was prepared by multiple post-grafting of alumina onto uniform mesoporous SiO2, which was assembled from monodisperse SiO2 microspheres. Hydrodesulfurization (HDS) catalyst was prepared by loading Ni and Mo active components onto the aluminated uniform mesoporous SiO2, and its HDS catalytic performance was evaluated using hydrodesulfurization of dibenzothiophene as the probe reaction at 300 C and 6.0 MPa in a tubular reactor. The samples were characterized by N2 physisorption, scanning electronic microscopy, Fourier transform infrared spectrum, X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH3-TPD), 27Al nuclear magnetic resonance (27Al-NMR) and high-resolution transmission electron microscopy (HRTEM). The results showed that the Si-OH group content of SiO2 was mainly dependent on the pretreatment conditions and had significant influence on the activity of the NiMo catalyst. The surface properties of the aluminated SiO2 varied with the Al2O3-grafting cycles. Generally after four cycles of grafting, the aluminated SiO2 behaved like amorphous alumina. In addition, plotting of activity of NiMo catalysts supported on aluminated mesomacroporous silica materials against the Al2O3-grafting cycle yields a volcano curve.
Aluminum grafting; Hydrodesulfurization; Surface properties; Catalyst support; SiO2
The demand for more environmentally friendly petroleum
products with lower sulfur content is growing due to
environmental problems caused by SOx emissions.
Therefore, development of new catalysts with high
hydrodesulfurization (HDS) activity is required. Previous
investigations (Breysse et al. 2003, 2008) indicate that the
nature and characteristics of catalyst support have
significant influence on the performance of HDS catalyst.
Generally, for industrial NiMo and CoMo HDS catalysts, the
supports are usually c-Al2O3 or aluminosilicates rather than
pure SiO2, mainly because of the stronger support?metal
interaction and subsequent better HDS activity of
Al2O3supported catalyst than that of SiO2-supported catalyst
(Scheffer et al. 1988). Ordered mesoporous Al2O3 is
particularly suited as catalyst support due to its suitable
surface and textural properties (Venezia et al. 2010; Morris
et al. 2008). Therefore, a number of researchers have aimed
to directly synthesize ordered mesoporous alumina
(Bagshaw and Pinnavaia 1960; Yada et al. 1997; Cabrera et al.
1999; Ma?rquez-Alvarez et al. 2008). However, the ordered
mesoporous alumina obtained by direct synthesis has some
shortcomings like the complexity of the synthesis process,
lack of reproducibility, low thermal stability at high
temperature, structural non-uniformity and wide pore
distribution (Cheralathan et al. 2008). Fortunately, one
alternative route to prepare ordered mesoporous alumina is
available, that is, grafting alumina onto ordered SiO2
mesoporous materials. This alternative method has distinct
advantages over direct synthesis with respect to
reproducibility, structural ordering and thermal stability
(Mokaya and Jones 1999; Mokaya 1997).
Up to now, various pure silica mesoporous materials,
such as MCM-41 (Goldbourt et al. 2002; Landau et al.
2001), SBA-15 (Zukal et al. 2008; Baca et al. 2008) and
KIT-1 (Ryoo and Kim 1997; Jun and Ryoo 2000), have
been employed as templates in many investigations
concerning alumination of SiO2 surfaces through grafting
technology. Several researchers have investigated the HDS
performance of catalysts supported on alumina-grafted
materials. Klimova et al. (2003) found that a catalyst with a
higher content of alumina has lower HDS activity, and
concluded that over strong metal?support interaction,
caused by incorporation of alumina in MCM-41, made the
reduction and sulfidation of the active component more
difficult. However, in the study of the effect of
incorporation of alumina on HDS activity of NiMo/SBA-15
catalysts, it was found that the blockage of catalyst pores by
alumina could lead to significant decline in activity in the
case of high content of alumina (Rayo et al. 2009). On the
one hand, owing to the small pore size of these molecular
sieves, the pore structure probably suffered from the
incorporation of alumina. On the other hand, the surface
property is significantly affected by the incorporation of
alumina. Therefore, it is difficult ascertain which is the
main cause for the decrease in HDS activity. Fortunately,
our research group has prepared SiO2 opal by ordered
facecentered cubic packing of monodisperse silica
microspheres (Zhou et al. 2012a, b). The ordered compact of
SiO2 opal with a high coordination number has very
uniform pore size distribution, and the pore size can be tuned
according to the needs of experiments by changing the
diameter of the monodisperse SiO2 microspheres. Hence,
SiO2 opals with larger pore size are more suitable materials
for the grafting of multilayer alumina, since larger pore
sized opal has better pore structure stability against the
amount of grafted alumina, than MCM-41 or SBA-15. In
addition, the content of silanol groups on the surface of
template porous SiO2 opal is supposed to have a great
influence on the post-synthesis of alumination (Mokaya
and Jones 1999; Li et al. 2001; Zhao et al. 1997), and the
content of the silanol groups mainly depends on
pretreatment conditions, such as calcination temperature and
whether or not there has been a hydrothermal treatment.
However, very few researchers have aimed to investigate
the effect of the content of silanol groups of template
porous SiO2 on HDS activity of catalyst using aluminated
SiO2 as catalyst support.
In the present work, the effects of pretreatment
conditions of the SiO2 materials and the Al2O3-grafting cycle on
the surface properties of aluminated SiO2 opal materials
were investigated. In order to ascertain the relationship
between the HDS activity and the surface property of the
aluminated SiO2 used as catalyst support, the catalytic
performance of NiMo catalysts supported on these
aluminated SiO2 opal materials was tested using the HDS of
dibenzothiophene (DBT) as probe reaction.
2.1 Preparation of SiO2 opal materials
and reference alumina
Monodisperse SiO2 microspheres were synthesized through
hydrolysis and condensation of tetraethyl orthosilicate
(98%, J&K) in alcohol (99.7%, Sinopharm Chemical
Reagent) and in the presence of water and ammonia by the
seed particle growth method. Detailed descriptions of the
synthesis procedures were reported in the previous papers
of our research group (Chen et al. 1996; Liu et al. 2009;
Chen 1998). In this work, monodisperse SiO2 microspheres
with diameters of 100, 250 and 500 nm were prepared.
First, SiO2 opals were obtained by ordered packing of SiO2
microspheres, then the assembled SiO2 opals were calcined
at three temperatures in the range of 500?900 C for 2 h,
and finally these SiO2 opals were hydrothermally treated at
220 C for 5 h to recover the surface silanol groups which
were lost during the calcination. The pretreatment of the
SiO2 opal includes the calcination and hydrothermal
treatment. In addition, a reference alumina, denoted as
(Al(NO3)3-C), was prepared by calcination of
Al(NO3)39H2O (99%, Sinopharm Chemical Reagent) in static air at
500 C for 5 h.
2.2 Alumination of SiO2 opal materials
In the alumination procedure, hydrothermally treated SiO2
opals were added to 0.67 M aluminum nitrate (Al(NO3)3)
solution at 80 C, kept in the solution for 12 h and then
filtered to remove the solution. To remove the Al(NO3)3
solution in the pores of the SiO2 opals, the filtered SiO2
opals were washed three times with distilled water. The
SiO2 opals were then dried at 100 C for 6 h and calcined
in static air at 500 C for 5 h. The above steps compose
one cycle of Al2O3 grafting, and the Al2O3-grafting process
can be repeated several times. The sample is denoted as
mSiO2-Al-n, where m is the diameter of the monodisperse
SiO2 microspheres, from which the SiO2 opal was made of,
and n is the number of Al2O3-grafting cycles. These
aluminated SiO2 opal materials were used as supports for the
2.3 Catalyst preparation
NiMo catalysts were prepared by incipient wetness
coimpregnation of aqueous solutions of (NH4)Mo7O2 4H2O
([99%, Sinopharm Chemical Reagent) and
Ni(NO3)26H2O ([98%, Sigma-Aldrich) into catalyst support. After
impregnation, the catalysts were dried at 100 C for 6 h
and then calcined at 500 C for 5 h. The active metallic
component loadings in the catalysts were 5.42 lmol
MoO3/m2 support and 1.81 lmol NiO/m2 support. For
comparison, the reference alumina was loaded with the
same loading amount of MoO3 and NiO by incipient
wetness co-impregnation of (NH4)Mo7O2 4H2O and
Ni(NO3)2 6H2O aqueous solutions. This catalyst was dried
and calcined as described above. All catalysts were crushed
and sieved to a size of about 0.23 mm before catalytic
2.4 Characterization of support and catalyst
The supports and catalysts were characterized by N2
physisorption, scanning electron microscopy (SEM), X-ray
diffraction (XRD), temperature-programmed desorption
(TPD), 27Al nuclear magnetic resonance (NMR) and
highresolution transmission electron microscopy (HRTEM). N2
adsorption/desorption isotherms were measured with a
Micromeritics ASAP 2020 automatic analyzer (ASAP
2020, Micromeritics, USA) at -196 C. Specific surface
areas were calculated by the BET method. The total pore
volume and pore size distributions were measured by the
mercury penetration method on an Autopore II 9220
mercury porosimeter using a contact angle of 140 . The
morphology of catalysts was observed on a scanning electron
microscope (Quanta 200F, FEI, USA) using an accelerating
voltage of 20 kV. HRTEM micrographs were obtained by
using a transmission electron microscope (JEM-2100,
JEOL, Japan) operated at 200 kV. The sulfided catalysts
were crushed and then ultrasonically dispersed in heptane,
and the suspension was collected on carbon-coated grids.
HRTEM micrographs were taken from different parts of
the same sample dispersed on the microscope grid.
Wideangle XRD patterns were recorded in the range of
10 \ 2h \ 85 on a Bruker D8 Advance diffractometer
(D8 Advance, Bruker, German), using Cu Ka radiation and
a goniometer speed of 1 (2h) min-1. The acid sites
amount and acid strength distribution were determined by
the NH3-TPD. The ammonia in the effluent gas was
detected by a thermal conductivity detector (TCD). Before
NH3-TPD experiments, the samples were pretreated in situ
at 550 C for 50 min in a N2 flow in order to remove water
and other contaminants. The samples were then cooled to
110 C, and a N2/NH3 mixture (30/10 mol/mol) at a flow
rate of 40 ml/min was fed for 30 min. The desorption step
was performed in a N2 stream (30 mL/min) at a heating
rate of 10 C/min. 27Al MAS NMR was carried out on a
Bruker AVANCE III 600 spectrometer at a resonance
frequency of 156.4 MHz using a 4 mm HX
double-resonance MAS probe at a sample spinning rate of 15 kHz. The
27Al chemical shift was referenced to 1 M aqueous
Al(NO3)3. 27Al MAS NMR spectra were recorded by the
small flip angle technique with a pulse length of 0.5 ls
(\p/12), a 1 s recycle delay and 4000 scans. Fourier
transform infrared spectroscopy (FTIR) was recorded on a
Bruker IFS 66 V spectrometer in the range of
3800?600 cm-1 (4 cm-1 resolution, 256 scans/spectrum)
using a Thermo Spectra-Tech high-temperature cell. All
the spectra were recorded at 150 C in argon after 2 h of
pretreatment at 450 C in argon.
2.5 HDS activity of NiMo catalyst
The HDS activity tests of the NiMo catalysts were carried
out in a bench-scale stainless-steel tubular reactor at
300 C and 6.0 MPa using a sulfur-free lube base oil
solution of DBT (1000 ppm of S) as HDS feedstock. The
lube base oil was provided by Sinopec Shanghai Gaoqiao
Petrochemical Corporation (China). Prior to catalytic HDS,
the catalysts were sulfided by a mixture of 2 wt% CS2 and
cyclohexane with the following temperature program:
reactor temperature was raised to 230 C from room
temperature at a heating rate of 7 C/min and kept at 230 C
for 3 h; then the temperature was raised to 340 C at a
heating rate of 3 C/min and kept at 340 C for 3 h. The
presulfiding conditions were as follows: liquid hourly space
velocity (LHSV), 5 h-1; H2/Oil (v/v) ratio, 200; operation
pressure, 6.0 MPa. After sulfidation, the stream was
switched from sulfiding agent to HDS feedstock and
maintained for 20 h to achieve catalyst stability, and the
hydrodesulfurized sample was collected at the appropriate
time (the time interval depended on the LHSV). Then, these
samples were washed with 5 wt% sodium hydroxide
solution three times to remove the dissolved H2S. The
sulfur content in the samples was measured by using a
THA-2000S UV-induced fluorescence sulfur analyzer
(Taizhou Jinhang Analytical Instruments Co. Ltd., China).
3 Results and discussion
3.1 Support and catalyst characterization
The pore structure characterization of supports and their
NiMo catalysts is shown in Tables 1 and 2, respectively. It
can be seen that the three SiO2 opal supports without
pretreatment had almost the same total pore volume (about
0.23 cm3/g), but their average pore diameter and specific
Table 1 Pore structure characterization of catalyst supports
Alumina content, g/100 m2 SiO2
Dm is the diameter of microspheres, nm; CT is the calcination temperature, C; HT is hydrothermal treatment; GC is the grafting cycles of
alumina; dp is the average pore diameter, nm; Vp is the pore volume, cm3/g; SSA is the specific surface area, cm2/g. Alumina content was
measured by EDX
Fig. 1 SEM images of NiMo catalysts with microspheres diameters of 100 nm (a), 250 nm (b), and 500 nm (c)
Table 2 Pore structure properties of NiMo catalysts
surface area were different from each other (Table 1). Due to
the shrinkage of the SiO2 microspheres during calcination
and sealing of the micropores on the surface of the SiO2
microspheres during hydrothermal treatment, the specific
surface area of SiO2 opal which was calcined and
hydrothermally treated had declined slightly. Additionally,
the pore structure did not change significantly after
successive Al2O3-grafting steps and NiMo impregnation (Tables 1,
2; Fig. 1), indicating that the well-defined pore structure of
the original SiO2 opals was still well maintained after several
cycles of Al2O3 grafting and NiMo impregnation. Figure 2
shows that catalysts were opal-like materials with
facecentered cubic packing of monodisperse nonporous spheres.
Fig. 2 Pore size distribution of four NiMo catalysts
Clearly, the well-defined pore structure was not damaged by
NiMo impregnation and Al2O3-grafting treatment.
The silanol group of the samples pretreated at different
conditions was evaluated by FTIR spectroscopy (Fig. 3).
According to the report by Brandriss and Margel (1993) the
absorbance at approximately 3750 cm-1 and at the range
approximately from 3265 to 3645 cm-1 was assigned to
Fig. 3 Infrared spectra for SiO2 opal, assembled from 100-nm
microspheres pretreated at different conditions
free silanol and chemisorption of water through silanol
group, respectively. Upon heating, the intensity of the
absorbance belonging to free silanol and the chemisorption
of water began to decrease, indicating that the surface
silanol group started to condense and eliminate water. After
hydrothermal treatment, the silanol group can be almost
completely recovered if the calcination temperature is
below 500 C; the silanol group could be partly recovered
if the calcination temperature above 500 C.
The 27Al NMR results of this work showed that the
pretreatment conditions of the SiO2 opals and the
alumination procedure had a strong effect on the Al coordination
(Fig. 4). It was reported that three distinct signals with
maximum values centered at 3, 34 and 56 ppm should be
assigned to octahedral Al (oct), pentahedral Al (pen) and
tetrahedral Al (tet) atoms, respectively (Zukal et al. 2008).
55 ppm 30 ppm 1 ppm
Fig. 4 27Al NMR spectra of SiO2 opal grafted with different amounts
of alumina and c-Al2O3
Fig. 5 NH3-TPD spectra of alumina-grafted materials, parent SiO2
opal and reference alumina
It can be seen that the chemical shift of the Al (tet) of the
grafted materials was closer to that of tetrahedral Al in
zeolite and amorphous aluminum silicate reported in
literature (Klimova et al. 2003; Hensen et al. 2010; Go?
raMarek et al. 2005), indicating the formation of Al?O?Si
bonds resulted from the grafting of Al atoms on the surface
of SiO2 microspheres. It is noteworthy that the spectra of
these Al2O3-grafted materials were different from those of
c-Al2O3, which consists mostly of Al(tet) and Al(oct) with
a Al(tet)/Al(oct) atomic ratio of 3/7, and the Al(pen)
accounts for very small amount. According to the report by
De Witte (De Witte et al. 1995), the Al(pen) usually exists
in the interface between silica and alumina or in the
amorphous silica?alumina, indicating that the local
arrangement of aluminum atoms in Al2O3-grafted materials
was different from that in bulk c-Al2O3. In addition, the
effect of calcination temperature on Al coordination is also
found from the 27Al NMR spectrum of A-700-H-2 and
A-900-H-2 in Fig. 4. The Al(tet) was formed through the
silanol group on the surface of the SiO2 microspheres;
therefore, the content of the Al(tet) mainly depended on the
amount of silanol group which was largely affected by the
calcination temperature. Based on the results of infrared
spectra (see Fig. 2), the content of silanol group of
A-700H is higher that of A-900-H, so more Al(tet) could exist
after two Al2O3-grafting cycles.
Further characterization of the supports was undertaken
by NH3-TPD to investigate the acidity of support with
different amounts of alumina, and the result is shown in
Fig. 5. NH3 desorption peaks at 120?220, 220?400 and
400?550 C represent weak, medium and strong acid,
respectively, and the acidity data of them are summarized
in Table 3. Table 3 shows that the pure silica support
presented mostly weak acid sites originating from the
Table 3 Acidity density of the parent SiO2 opal, grafted materials and reference alumina
Acidity density, lmol NH3/m2
Weak acid 110?220 C
Medium acid 220?400 C
Strong acid 400?600 C
silanol group on the surface, and only a small amount of
medium and strong acid sites. However, the amorphous
alumina (Sample Al(NO3)3-C) presented mostly medium
and strong acid sites. After two cycles of alumina grafting
(Sample A-700-H-2), the alumina-grafted materials
showed a significant decrease in the amount of weak acid
sites, indicating that the silanol group was consumed in the
reaction with aluminum species, and medium and strong
acid sites were created by Al2O3 grafting. After four cycles
of alumina grafting (Sample A-700-H-4), the total acidity
decreased, possibly due to the formation of more
amorphous alumina like the sample Al(NO3)3-C. Based on the
result of the 27Al NMR (Fig. 4), it can be concluded that
the total acidity increased first and then decreased with the
grafting cycle, confirming that some of the grafted alumina
atoms would be bonded with silica atoms of the silica
microspheres after two-cycle grafting, but the acid sites
would be covered by the alumina overlayer during the
subsequent Al2O3 grafting. By comparing the NH3-TPD
spectra (Fig. 5) of the A-700-H-2 and A-900-H-2 and
infrared spectra (Fig. 3) of the A-700-H and A-900-H, it
can be found that higher content of silanol group is more
conducive to forming acid sites during the grafting
The supports and their corresponding NiMo catalysts
were analyzed by XRD, and the results are shown in Fig. 6.
No distinct diffraction peaks are observed in the XRD
patterns of Al(NO3)3-C (Fig. 6a), indicating that
amorphous alumina was formed during calcination of Al(NO3)3
at 500 C. The XRD patterns of SiO2 opal showed a broad
signal in the 2h range between 15 and 35 (samples A in
Fig. 6a), and this was attributed to the amorphous silica
(Zepeda et al. 2008; Nava et al. 2007). It is suggested that
the alumina unbonded with silicon atoms of SiO2
microspheres can be regarded as amorphous alumina similar to
the Al(NO3)3-C sample. As shown in Fig. 6b, no
reflections belonging to molybdenum and nickel oxides are
observed in the XRD patterns of the NiMo/Al(NO3)3-C
catalyst. This result indicated a good dispersion of
deposited Ni and Mo oxide species on the support surface. For
NiMo/SiO2 catalyst, besides the peaks from the support,
some peaks of molybdenum oxide appeared at 2h of 12.7 ,
Fig. 6 XRD patterns for supports (a) and corresponding NiMo
22.7 and 33.4 (JCPDS (Joint Committee on Powder
Diffraction Standards) card 35-609). An additional
reflection peak appeared at 2h of 25.6 , indicating the presence
of b-NiMo4 phase (JCPDS card 21-0868). However, in the
case of NiMo/Al2O3-grafted SiO2 catalyst, no reflections of
MoO3 and b-NiMo4 are found, indicating that the
incorporation of alumina in the supports enhanced the dispersion
of active components of the catalysts.
3.2 Catalytic activity in hydrodesulfurization
In the present study, the catalytic activity of NiMo catalyst
using alumina-grafted SiO2 as support was tested using the
HDS of dibenzothiophene (DBT) in a fixed-bed tubular
reactor as the probe reaction. The HDS catalytic activity of
NiMo catalysts supported on parent SiO2 opals, and
amorphous alumina was also tested for comparison
purposes. Surface area hourly space velocity (SHSV) was
applied to describe the ratio of flow rate of liquid feedstock
to the catalyst surface area. According to the literature
(Chen et al. 2005; Chen and Ring 2004), HDS reactions of
individual compounds follow first-order kinetics, so the
material balance of the isothermal and plug-flow reactor is
given by Eq. (1).
where C0 and Ct is the sulfur concentration in the feedstock
and product stream, respectively, ppm; k is the
pseudofirst-order reaction rate constant, m s-1; Q is the feedstock
flow rate, m3 s-1; S is the surface area of the catalyst
loaded in the reactor, m2.
For the HDS catalytic performance testing, SHSV was
varied through changing the Q, the liquid effluents from the
reactor were collected, and their sulfur content was
measured. The relationship between ln(C0/Ct) and SHSV-1 is
shown in Fig. 7. The best fit straight lines of ln(C0/Ct)
versus SHSV-1 were obtained, and the slope of the straight
line was the k. Three SiO2 opals, prepared with 100, 250
and 500 nm SiO2 microspheres, were employed as
templates to fabricate NiMo catalysts. The k of various NiMo
catalysts supported on SiO2 opal grafted with different
amounts of Al2O3 is calculated and presented in Fig. 8, and
the n in the legend at the top left corner represent the
Al2O3-grafting cycles. It can be found that the catalytic
activities of all the NiMo catalysts supported on aluminated
SiO2 opals were significantly higher than those of NiMo
catalysts supported on parent SiO2 opals. In addition, the
activity of NiMo catalysts supported on the aluminated
SiO2 yielded a volcano curve as a function of the
Al2O3grafting cycles. According to the results of 27Al NMR in
Fig. 4, a certain amount of Al was implanted within the
outer surface of the silica microspheres during the grafting
Fig. 8 Reaction rate constant k of various NiMo catalysts supported
on Al-grafted materials with different amounts of alumina (300 C,
6.0 MPa, H2/Oil (v/v) = 800)
process, and amorphous aluminum silicate was formed.
Then the content of amorphous aluminum silicate
gradually increased with the grafting cycles. Therefore, the
activities of the catalysts supported on aluminated SiO2
began to increase till it reached a maximum. The maximum
k of the three NiMo catalysts was 5.64 m s-1 for
NiMo/A700-H-2, 6.32 m s-1 for NiMo/B-700-H-2 and 7.60 m s-1
for NiMo/C-700-H-2, respectively. Excessive Al grafting
resulted in the formation of amorphous alumina that caused
a significant decrease in k. As a result, the k of the three
NiMo catalysts supported on the SiO2 with four
Al2O3grafting cycles was almost equal to that of NiMo catalyst
Fig. 7 Relationship between SHSV-1 and ln(C0/Ct) (300 C,
6.0 MPa, H2/Oil (v/v) = 800)
Fig. 9 Effect of pretreatment conditions on the reaction rate constant
k of NiMo catalysts (300 C, 6.0 MPa, H2/Oil (v/v) = 800)
supported on Al(NO3)3-C. Several researchers (Landau
et al. 2001; Rayo et al. 2009) have reported the catalytic
activity of catalysts supported on Al2O3-grafted materials,
and they found that when the content of the alumina
exceeds a certain level, the activity of the catalysts began to
decrease significantly. In their studies, molecular sieves
were used, and the pore textural properties of molecular
sieves were prone to being damaged when a high content of
alumina is introduced. Therefore, the decrease in the
activity may be attributed to this damage of the textural
properties. In the present research work, the textural
properties of the aluminated SiO2 changed only slightly
with the grafting cycles (see Tables 1, 2), suggesting that
the decrease in the catalyst activity was resulted from the
difference in surface property rather than textural structure.
The supports pretreated at different conditions were
used to prepare NiMo catalysts, and the effect of
pretreatment conditions on the catalytic activity was
evaluated. It can be found that the NiMo/A-700-H-2 catalyst
showed significantly higher HDS activity compared to the
NiMo/A-700-2, indicating hydrothermal treatment can
increase the catalyst activity through the recovery of
silanol groups of the SiO2 spheres. As shown in Fig. 9,
the catalyst activity increased with a decrease in
calcination temperature and declined greatly when the
calcination temperature reached 900 C. This was because the
silanol groups lost during calcinations, especially at
higher temperature, were just partly recovered in the
hydrothermal treatment procedure, and the higher the
calcination temperature, the more the silanol groups on
the silica spheres were lost.
As shown in Fig. 10, the size and the dispersions of
sulfided Mo species (MoS2) of different supports were
observed by using HRTEM. The size of active component
MoS2 on SiO2 support is large and its dispersion is bad in
Fig. 10a, in comparison with that in Fig. 10b, due to the
weak interaction between metal oxide and SiO2 support
(Scheffer et al. 1988), resulting in the low reaction rate
constant k (see Fig. 8). With the alumination of SiO2
supports, the dispersion of sulfided Mo species was
improved significantly (Fig. 10b), and this is in accordance
with the XRD results (see Fig. 6). It was suggested that the
support, which is able to disperse the active components
well, has relatively high catalytic activity.
NiMo catalysts supported on SiO2 opals aluminated by
grafting with Al2O3 were prepared and characterized, and
their HDS catalytic activity was tested. It was shown with
the increase in grafted alumina content on the surface of
SiO2, the surface properties changed from SiO2 to
amorphous aluminosilicate and then to amorphous alumina. In
line with this, the HDS rate constant k of NiMo catalysts
supported on the aluminated SiO2 followed a volcano curve
when plotted as a function of alumina content. The
alumination process of SiO2 opals by chemical grafting did
not affect their well-defined pore structure, and alumination
of the SiO2 surface led to an increase in the metal?support
interaction and improved the dispersibility of the active
species on the support surface.
Fig. 10 HRTEM images of NiMo catalysts supported on a A-700-H, b A-700-H-2
Acknowledgements Financial support by the National Natural
Science Foundation of China (Grant No. 91534120) and the Shanghai
Petrochemical Company of Sinopec (under the contract number
30450127-13-ZC0607-0001) is greatly acknowledged.
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