Peptization Mechanism of Boehmite and Its Effect on the Preparation

Article
pubs.acs.org/IECR
Peptization Mechanism of Boehmite and Its Effect on the
Preparation of a Fluid Catalytic Cracking Catalyst
Yongsheng Zheng,† Jiaqing Song,*,† Xiangyu Xu,† Mingyuan He,‡ Qian Wang,§ and Lijun Yan§
†
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200241, China
§
Petrochemical Research Institute of Petrochina, Beijing 100195, China
‡
ABSTRACT: A peptization mechanism of boehmite was proposed in this work. The effects of peptization on the properties and
performances of a fluid catalytic cracking (FCC) catalyst were explored. A boehmite colloid was prepared through peptization of
boehmite with hydrochloric acid, and its particle size distribution was characterized during peptization. With an increase of the
acid/alumina molar ratio from 0 to 0.11, the particle size of the boehmite colloid decreased to 3.5 nm. The particle size increased
from 3.5 to 11 nm when the acid/alumina molar ratio was further increased to 0.16 and remained at 10 nm thereafter. The
smallest dispersed boehmite particles were obtained in an acid/alumina molar ratio of 0.11. On the basis of many experiments,
the dispersion and coalition mechanisms of boehmite during acid peptization were proposed. Boehmite particles absorb protons
on the surface hydroxyl groups and repel each other to form colloidal particles when the acid/aluminia ratio is low. With more
hydrochloric acid added, large amounts of chloride anions cause compression of the boehmite diffuse layer, thus resulting in
boehmite coalition. A FCC catalyst was prepared by peptizing boehmite with an acid/alumina molar ratio of 0.11. The catalyst
has a larger external surface area, a higher mesoporous volume, and better acidity distribution than the catalyst prepared with
boehmite. Both conversion of residue oil and yields of diesel and gasoline over a peptized catalyst are higher than those over the
catalyst without being peptized.
1. INTRODUCTION
In this paper, boehmite colloids were prepared by peptization
of boehmite with hydrochloric acid under various conditions. A
peptization mechanism of boehmite, including dispersion and
coalition, was proposed. In addition, a FCC catalyst was
prepared with peptized boehmite. The effects of boehmite
peptization on the preparation and performance of a FCC
catalyst were discussed based on the fix-fluidized bed, nitrogen
adsorption−desorption characterizations, and Fourier transform infrared (FTIR) measurements.
Colloidal boehmite prepared by peptization of boehmite with
mineral acid has been widely used for the fabrication of ceramic
powders,1−4 hollow microspheres,5 porous membranes,6
dispersants,7,8 and photoluminescent and mesoporous alumina,9,10 pollution control,11 separation technology,12,13 paste,14
and so on. As a matrix and binder of a fluid catalytic cracking
(FCC) catalyst, peptized boehmite can promote its catalytic
activity for gas oil and residual oil cracking.15,16 In 2008, the
catalytic cracking capacity was up to 600 million tons/year,
which is about 23% of the total refining capacity in the
petroleum industry.17 With increasing tension over and inferior
quality of oil resources, the residual oil cracking ability of FCC
catalysts needs to be improved a great deal. Currently, residual
oil with a dynamic molecular diameter of 1−2 nm and a boiling
point above 500 °C cannot be cracked in zeolite pores, so a
FCC catalyst with larger surface area and better pore volume
distribution is required.
Because the properties of peptized boehmite in a spray dryer
slurry of a FCC catalyst are difficult to observe, the mechanism
of boehmite peptization has been studied individually by
various parameters such as the viscosity,18,19 particle size
distribution,20,21 acid ratio,22 phase diagram,23 anion type,
anion concentration, alumina concentration, peptization time
and temperature,24 dispersion,25 surface charges, sedimentation,26,27 nature of the colloid boehmite by NMR,28 etc. Among
them, the acid/alumina molar ratio ([HCl]/[Al2O3]), ζ
potential, and particle size distribution are the most widely
used to study the properties of colloidal boehmite because of
the simplicity and intuition of these methods.
© 2014 American Chemical Society
2. EXPERIMENTAL SECTION
2.1. Materials. Chemicals used in this work, including
hydrochloric acid (HCl; 36−38 wt %, AR), potassium chloride
(AR), and ammonium dihydrogen phosphate (AR), were
purchased from commercial vendors without further purification. Other chemicals, including zeolite REY, zeolite USY,
alumina sol, and kaolin clay, were purchased from a commercial
vendor (Shanxi Tengmao Technology Ltd.). The particle size
of zeolites and kaolin clay is about 1 μm. The surface area and
pore volume of boehmite (Shandong Aluminum Industry Co.,
Ltd.) are 332.92 m2/g and 0.29 cm3/g.
2.2. Boehmite Peptization Process. In a typical experiment, 7.74 g of boehmite (64.64 wt %) was mixed with 41.30 g
of deionized water, and 0.97 g of HCl (18.56 wt %) was added
dropwise to the solution under vigorous stirring at room
temperature. The maximum acid/aluminum molar ratio was
Received:
Revised:
Accepted:
Published:
10029
March 12, 2014
May 13, 2014
May 24, 2014
May 25, 2014
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boehmite. The t-plot external surface area was measured by a
TriStar II 3020 isothermal adsorption instrument. The primary
particle size was calculated according to the following equation:
R = 3000/Sρ, where R is the primary particle size of boehmite,
S is the t-plot external surface area, and ρ is the unit cell density
of boehmite, which is 3.014 g/cm3 here.
kept below 0.3. The pH of the mixture was then measured with
a pH meter. The mixture was then centrifuged, and the particle
size of the supernatant colloid was analyzed on a laser particle
analyzer.
2.3. Catalyst Preparation. The FCC catalyst A was
prepared with peptized boehmite. Briefly, zeolite REY, zeolite
USY, peptized boehmite (PB), alumina sol (A), kaolin clay
(KC), and deionized water were mixed and stirred for 30 min.
The dry weight ratios of each reactant are shown in Table 1,
3. RESULTS AND DISCUSSION
3.1. Dispersion of Boehmite. The diffusion rate of
protons into particles (Rp) and in the solution (Rs) and the
adsorption rate of protons on the surface of boehmite (Ra)
were defined as shown in Figure 1. For convenience, the
Table 1. Dry Weight Ratios of the Materials Mixed
dry weight ratio (wt %)
REY
USY
PB
A
KC
18
10
18
3
51
and the solid content was 40 wt %. Under stirring, a certain
amount of HCl (18.56 wt %) was dropwise added to the
mixture and resulted in a [HCl]/[Al2O3] molar ratio of 0.11.
The mixture was then spray-dried, calcined at 540 °C for 2 h,
ion-exchanged by ammonium dihydrogen (5 wt %) phosphate,
and hydrothermally aged for 17 h. Catalyst B was prepared with
the same procedure except that boehmite instead of peptized
boehmite was used.
2.4. Characterizations. The particle size and ζ potential
were tested on a Zetasizer Nano ZS 90 laser particle analyzer
(Malvern Instruments Ltd.). The pH values were measured
with a Sartoruis PB-10 pH meter (Sartorius Stedim Biotech
GmbH). Transmission electron microscopy (TEM) studies
were carried out on a JEM-2100 electron microscope. Nitrogen
adsorption−desorption characterizations were done by a
TriStar II 3020 isothermal adsorption instrument. The
parameters of a fixed fluidized bed are listed in Table 2.
Figure 1. Scheme for the diffusion and adsorption rates of protons.
magnitudes of Rp and Rs are only determined by the
concentration of protons in solution, and the magnitude of
Ra is controlled by the concentration of protons and the
amounts of boehmite surface hydroxyl groups. Boehmite
colloids were prepared by adding HCl into the boehmite
solution either under stirring or static. As shown in Table 3, the
way of adding HCl had no effect on the particle size of
boehmite. This might be caused by the fact that Rs is much
faster than Rp and Ra.
Table 2. Parameters of a Fixed Fluidized Bed
item
parameter
feedstock oil
catalyst weight (g)
temperature (°C)
catalyst/oil weight ratio
WHSVc (h−1)
material balanced (%)
Daqing FCC feedstock oil (ARa) + 45VRb
200
460−530
6 or 8
15
95−105
Table 3. Effect of the Ways of Adding HCl on the Particle
Size of Boehmite Colloids
Atmospheric residue. bVacuum residue. cWeight hourly space
velocity = feed speed (g/h)/catalyst weight (g). dMaterial balance =
total product weight/feedstock weight.
acid alumina molar ratio
particle size (stirring)/
nm
particle size (static)/
nm
0.01
0.04
6.5
31.1
6.6
30.5
FTIR analysis was carried out using pyridine as the probe
molecule. About 16 mg of a solid sample was finely ground,
tableted, and heated to 450 °C, followed by evacuation under
ca. 10−3 Pa for 2 h. The sample was then allowed to absorb
pyridine at 90 °C for 30 min, evacuated for 30 min at 200 °C,
and cooled to room temperature before FTIR analysis on a
Nicolet 6700 Fourier transform spectrometer.
2.5. Primary Particle Size Measurement. The primary
particle size was determined using the X-ray diffraction (XRD)
method and t-plot external surface area. XRD measurement was
conducted on a Bruker D8 Advance diffractometer with Cu Kα
radiation. The Scherrer equation R = 0.45λ/(β cos θ) was used
to calculate the primary particle size, where R is the primary
particle size of boehmite, λ is the X-ray wavelength, which is
equal to 0.154 nm, β is the width of the peak [full width at halfmaximum (fwhm)] after correcting for the instrumental peak’s
broadening, and θ is the Bragg angle of the (020) peak for
Boehmite particles are the secondary particles that consist of
numerous primary particles. In order to illustrate the dispersion
of boehmite particles, the term of dispersed particles is defined
as the boehmite particles dispersed. TEM images of the
boehmite particles in the supernatant colloid before and after
peptization are shown in Figure 2. Before the acid was added,
the secondary particles aggregated together and showed a tight
structure (Figure 2a). The boehmite particles were dispersed
from the secondary particles to form small dispersed particles
when the acid/alumina molar ratio increased to 0.02 and 0.11
(Figure 2b,c). To further explore the relationship between the
boehmite particle dispersion and acid/alumina molar ratio,
various amounts of HCl were added and size distributions of
the dispersed particles were analyzed. Also, the dispersion of
the boehmite particles affected by ammonium dihydrogen
phosphate was investigated to understand the influence of the
surface hydroxyl groups.
a
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Figure 2. TEM pictures of colloid particles at acid/alumina molar ratios of (a) 0, (b) 0.02, and (c) 0.11.
of 0.11. The ζ-potential measurement shows that the repulse
force between boehmite particles increased when the acid/
alumina molar ratio was increased from 0 to 0.07. On the basis
of these results, it can be determined that dispersion of
supernatant colloidal particles happened in stage 1. A possible
explanation of this boehmite particle dispersion is shown in
Figure 5. With HCl added, the secondary boehmite particles
Figure 4 shows the effect of the acid/alumina molar ratio on
the particle size distribution. The size change of the dispersed
Figure 3. XRD pattern of boehmite.
Figure 5. Scheme for boehmite peptization from (A) a secondary
boemite particle to (B−D) dispersed boehmite particles.
absorb protons and repulse each other because of electrostatic
repulsion force. The higher the acid/alumina molar ratio is, the
smaller the dispersed particles are. The absorption of protons
on the particle surface causes a decreased proton concentration
inside the dispersed or boehmite particle, which resulted in
decreasing Rp and Ra. Rp and Ra determine the uniformity and
size of the dispersed particles, respectively.
The influence of dihydrogen phosphate ions on the particle
size was investigated by adding ammonium dihydrogen
phosphate to the boehmite solution before peptization. The
phosphorus/aluminum molar ratios varied from 0 to 0.1, and
the acid/aluminum molar ratio remained at 0.11. The particle
sizes in the phosphorus/aluminum molar ratio range are listed
in Table 4. Possible reactions occurred as follows when
Figure 4. Effect of the acid/alumina molar ratio on the particle size
distribution and ζ potential by a laser particle analyzer.
particles went through three stages with increasing acid/
alumina molar ratio. In stage 1, where the acid/alumina molar
ratio was increased from 0 to 0.11, the particle size decreased
dramatically to 3.5 nm. According to XRD analysis (Figure 3)
and the t-plot external surface area method, the primary particle
sizes of boehmite are 3.99 nm (β = 0.018 rad; θ = 14.19°) and
2.99 nm (the t-plot surface area of boehmite is 332.92 m2/g and
pore volume is 0.29 cm3/g), respectively, indicating that the
boehmite secondary particles were dispersed to single primary
particles. In stage 2, where the acid/alumina molar ratio was
increased from 0.11 to 0.16, the particle size increased from 3.5
to 11 nm. When the acid/alumina molar ratio was further
increased in stage 3, the particle size remained at 10 nm. The
smallest particle size was found in an acid/alumina molar ratio
Table 4. Particle Size of Boehmite with Ammonium
Dihydrogen Phosphate at an Acid/Alumina Ratio of 0.11
and Without Ammonium Dihydrogen Phosphate at Various
Acid/Alumina Ratios
10031
phosphorus/aluminum
ratio
particle size/
nm
acid/aluminum
ratio
particle size/
nm
0
0.05
0.06
0.08
0.10
3.5
15.2
27.8
63.2
391.9
0.11
0.0125
0.01
0.005
0
3.5
16.2
30.5
49.7
396.6
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Industrial & Engineering Chemistry Research
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compresses and the interface of the diffusion and compact
layers reduces to the original compact layer.29 The electrostatic
repulsion force of dispersed particles is consequently reduced.
The Brownian movement of the particles dominates their
moving behaviors; thus, the dispersed particles collide with
each other and aggregate.
Although the particle size increased from 4.9 to 3.5 nm with
increasing acid/alumina ratio from 0.07 to 0.11, the ζ potential
decreased from 49.1 to 44.3 mV because of an increase of the
chloride ion concentration (Figure 4). The further increasing
chloride ion concentration causes an increase of the particle size
and a decrease of the ζ potential, and eventually the colloidal
solution gelled with acid/alumina molar ratios higher than 0.15.
The ζ potential could not be tested in situ at this point.
3.3. Effect of Peptization on the Preparation of a FCC
Catalyst. As discussed above, the process of peptization
includes two stages, e.g., dispersion and coalition. However,
how peptization affects the preparation of a FCC catalyst is still
unclear. Because the peptization abilities of different boehmites
vary depending on the properties of boehmite,30 only general
effects of boehmite peptization on a FCC catalyst will be
discussed in this work. Catalyst A was prepared by peptization
of boehmite with a [HCl]/[Al2O3] ratio of 0.11 in the smallest
particle distribution. Catalyst B was prepared without
peptization. The performances of a FCC catalyst were
evaluated via nitrogen adsorption−desorption, FTIR analysis,
and characterization of a fixed fluidized bed.
The performances of catalysts A and B were tested with a
catalyst/oil weight ratio of 6 at 490 °C. The product yields are
listed in Table 7. It can be seen that catalyst A gives higher
ammonium dihydrogen phosphate was added to the boehmite
solution: AlOH + H2PO4− = AlHPO4− + H2O. It can be
concluded that the outer surface hydroxyl groups were partially
covered by dihydrogen phosphate ions, which could inhibit the
absorption of protons to the outer surface of boehmite particles
thereafter. Therefore, Rp increased, Ra decreased, and the
dispersed particles became larger and more uniform. When the
repulse force between adsorbed protons was insufficient to
break boehmite particles, the particle size of boehmite remained
391.9 nm. In full, the particle size of dispersed boehmite was
determined by the acid/alumina ratio and surface hydroxyl
group distribution.
3.2. Coalition of Boehmite. Coalition of boehmite
happened in stage 2 (Figure 4), where the size particles
increased slightly with an increase of the acid/alumina molar
ratio. To explore the underlying mechanism of this coalition,
the following two experiments were conducted.
Colloid prepared with a fixed acid/alumina molar ratio of
0.15 was gradually diluted with deionized water. As can be seen
in Table 5, the particle size and ζ potential remained steady
Table 5. Particle Size and ζ Potential of Diluted Supernatant
Colloids at Different pH Values
pH value
particle size/nm
ζ potential/mV
1.59
1.84
1.99
2.27
2.57
5.3
5.6
6.4
6.0
6.7
33.8
36.2
38.0
46.1
47.8
when the pH was changed from 1.57 to 2.57. This is caused by
the steadiness of the protons absorbed on the boehmite particle
surface.
Chloride ions were added to the boehmite solution to get
chlorine/alumina molar ratios of 0.15 and 0.2 in the colloid.
The acid/alumina molar ratio was kept at 0.1. As shown in
Table 6, the paricle size increased with an increase of the
Table 7. Product Yields of Catalysts A and B at WHSV of 15,
Catalyst/Oil Weight Ratio of 6, and Temperature of 490 °C
Table 6. Particle Size of Boehmite at Chlorine/Alumina
Molar Ratios of 0.15 and 0.2 and Acid/Alumina Molar
Ratios of 0.15 and 0.2
a
chlorine/alumina
ratio
particle size/
nm
acid/alumina molar
ratio
particle size/
nm
0.15
0.2
6.9
9.9
0.15
0.2
6.4
11.0
product
catalyst A yield (wt %)
catalyst B yield (wt %)
dry gas (wt %)
LPG (wt %)
gasoline (wt %)
diesel (wt %)
heavy oil (wt %)
coke (wt %)
conversiona (wt %)
1.46
9.31
46.34
26.02
9.50
7.37
64.48
2.04
12.25
42.44
23.91
10.28
9.08
65.82
Conversion = dry gas + LPG + gasoline + diesel + coke.
yields of high-value products, including gasoline and diesel,
than catalyst B. On the contrary, catalyst A produces fewer lowvalue products, such as dry gas, LPG, coke, and heavy oil, than
catalyst B. The performances of catalysts A and B were tested
with catalyst/oil weight ratios of 6 and 8 at various
temperatures, as shown in Figure 7. It is clear that the yields
of gasoline and diesel over catalyst A are higher than those over
catalyst B at the same conversion rate. These results indicate
that more feedstock is converted into valuable product and
chloride ion concentration. This indicates that chloride ions
instead of protons contributed to the size increasing in stages 2
and 3. On the basis of these results, a possible mechanism of
aggregation of the dispersed boehmite particle in stages 2 and 3
is proposed, as shown in Figure 6. In an environment with a
high amount of anions, the diffuse layer of boehmite
Figure 6. Influence of the anions on coalition of boehmite.
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Figure 7. Fixed fluidized bed results of catalysts A and B.
Figure 8. Pore size distributions of catalysts A and B.
better product distribution is acquired using catalyst A at the
same WHSV values.
As mentioned above, the diameter of residual oil is between 1
and 2 nm. According to the theory proposed by Spry and
Sawyer, the desired diameter of the catalytic sites should be
about 10−20 times the size of the diffusing molecules. Thus, a
large external surface area and a pore volume of about 10−40
nm are required to get the desired conversion rate of residual
oil and high yields of diesel and gasoline.17
The BET and t-plot results of catalysts A and B are listed in
Table 8. Catalyst A has almost the same total surface, total pore
Table 8. BET and t-Plot Results of Catalysts A and B
catalyst A catalyst B
BET (g/m2)
t-plot micropore surface area (g/m2)
t-plot external surface area (g/m2)
single-point adsorption total pore volume of the pores
(cm3/g)
t-plot micropore volume (cm3/g)
BJH adsorption cumulative volume of the pores
between 17.000 and 3000.000 Å diameter (cm3/g)
103.69
24.21
79.49
0.18
102.00
44.76
57.24
0.17
0.005
0.16
0.014
0.16
Figure 9. IR spectra of pyridine-adsorbed catalysts A and B.
4. CONCLUSIONS
Boehmite was subjected to dispersion and coalition during
peptization with HCl. The mechanism of these phenomena was
elucidated through a series of simple experiments. The
boehmite particles were dispersed by the electrostatic repulsion
between the protons on their surface absorbing to the surface
hydroxyl groups. When a high acid/alumina ratio is used, the
high concentration of anion from the acid can cause boehmite
particle aggregation. A FCC catalyst prepared with peptized
boehmite has a larger external surface area, a mesoporous
volume, and a better zeolite acidity distribution, which can
improve the residue oil conversion and yields of gasoline and
diesel.
volume, and mesoporous volume as catalyst B. However,
catalyst A has a smaller micropore surface area and thus a larger
external surface area. Therefore, the zeolite activity in catalyst A
is lower than that in catalyst B. The high zeolite activity could
result in high coke and LPG yields, so the influences of HCl to
zeolite are effective to produce high-value products. The
mesoporous volume distributions of catalysts A and B are
shown in Figure 8. The mesoporous volume of catalyst A with
sizes ranging from 4 to 40 nm is twice higher than that of
catalyst B in the same size range. It can be explained that
dispersed boehmite particles in catalyst A form stacking pores
with other materials, while the bulk boehmite particles in
catalyst B sintered after hydrothermal aging, which could be
attributed to the cracking of residual oil.
As illustrated in Figure 9, the bands attributed to pyridine
molecules coordinated to Lewis acidity sites and pyridinium
ions formed by protonation of pyridine on Bronsted acidity
sites are observed at 1450 and 1540 cm−1, respectively.31 Thus,
it can be confirmed that both Lewis and Bronsted acidity sites
are diminished because of dealumination of zeolite after
peptization of catalyst A using HCl. Peptization gives a better
acidity distribution, which reduces the formation of coke and
cracking in gasoline and diesel products.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
We acknowledge National “Twelfth Five-Year” Plan for Science
& Technology Support (Grant 2012BAE05B00).
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