Study on the synthesis of organized mesoporous alumina in a rotating packed bed

Article abstract of DOI:10.1016/j.materresbull.2012.10.023

In this study, a novel precipitation technology was used to synthesize organized mesoporous alumina (OMA) in a rotating packed bed (RPB) by using aluminum nitrate and ammonium carbonate as the reactants and polyethylene glycol 1540 as the template. The properties of the as-prepared OMA were analyzed by BET, XRD and TEM. The effects of the synthesis conditions, surfactant and pore-expanding agent on OMA properties were explored, and the formation mechanism of OMA was discussed. The experimental results indicated that the excellent micromixing capability of the RPB resulted in the formation of OMA with narrow pore size distribution and uniform structure. It was also found that the high-gravity level, liquid flow rate and mixing time had significant effect on the pore structure of OMA.

Full text of DOI:10.1016/j.materresbull.2012.10.023

Materials Research Bulletin 48 (2013) 290–294
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Study on the synthesis of organized mesoporous alumina in a rotating packed bed
Zhisong Tang a,b, Fen Guo a,b, , Baochang Sun a,b, Xiangsen Wu a,b
*
a State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
b Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, China
A R T I C L E I N F O
A B S T R A C T
Article history:
In this study, a novel precipitation technology was used to synthesize organized mesoporous alumina
(OMA) in a rotating packed bed (RPB) by using aluminumnitrate and ammonium carbonate as the reactants
and polyethyleneglycol1540asthetemplate.Thepropertiesoftheas-preparedOMAwere analyzedbyBET,
XRD and TEM. The effects of the synthesis conditions, surfactant and pore-expanding agent on OMA
properties were explored, and the formation mechanism of OMA was discussed. The experimental results
indicated that the excellent micromixing capability of the RPB resulted in the formation of OMA with
narrowporesizedistributionand uniform structure. Itwasalsofoundthatthehigh-gravitylevel, liquid flow
rate and mixing time had significant effect on the pore structure of OMA.
Received 22 February 2012
Received in revised form 6 October 2012
Accepted 15 October 2012
Available online 23 October 2012
ß 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Rotating packed bed (RPB) is an effective intensification
technology, which has been applied to many processes including
The first successful synthesis of M41S mesoporous molecular
sieves by Mobil researchers [1,2] opened a new era in the research
of inorganic molecular sieves. This discovery exhibited exciting
possibilities for new types of molecular sieves with a significantly
higher surface area and a narrow pore size distribution. These
materials have huge potential in many industrial applications
including microelectric circuits, electrochemical devices, gas
sensors, fuel cells, absorption, and catalysis [3–5].
Organized mesoporous alumina (OMA) is a very interesting
molecular sieve [6]. Because of the remarkable features of OMA and
its potential applications in industry, many studies have been
directedtotheproductionofOMAwithhighsurfaceareaandnarrow
pore size distributions in the mesoporous range. Wu et al. [7]
presented the synthesis of mesoporous alumina materials with an
ordered 2D hexagonal structure, surface area of 309 m2/g, pore
volume of 0.51 cm3/g, pore size of 7.5 nm, and thermal stability of
900 8C. Deng et al. [8] reported the synthesis of mesoporous
aluminas by using the nonionic templating method, and investigat-
distillation [13], absorption [14], evaporative cooling [15], ozona-
tion [16] and precipitation [17]. It has been reported that RPBs can
greatly enhance mass transfer and micromixing, and is an enabling
precipitation technology for nanoparticles preparation with a good
control of particle size distribution [17,18]. We have reported the
mass production of mesoporous alumina by the precipitation
method in an RPB [19]. Inexpensive inorganic aluminum salts and
polyethylene glycol 1540 (PEG 1540) were used as the sources of
aluminum and template, respectively. The comparison of experi-
mental results between the RPB and a stirred tank, and the effects
of operating conditions including reactant concentration, addition
rate of precipitator on mesoporous structure were investigated in
the previous study. The results indicated that OMA with ordered
wormlike pores, narrow pore size distributions, and a surface area
of 250 m2/g was synthesized in an RPB. The study provided a
feasible technology for mass production of mesoporous alumina.
However, the effects of the synthesis conditions, surfactant and
pore-expanding agent on OMA properties and the formation
mechanism of OMA are unclear and deserve a further study.
Herein, we report the synthesis of OMA in an RPB, and the effects
of the operation conditions, surfactant and pore-expanding agent on
OMA properties were studied. Besides, the OMA formation
mechanism was analyzed according to micromixing theory.
ˇ
´
ed the characterization the mesoporous aluminas. Zilkova et al. [9]
presented the synthesis of organized mesoporous alumina by using
1-methyl-3-octylimidazolium chloride as a structure-directing
agent. Similar mesoporous alumina has also been synthesized
[10–12]. However, the above methods need expensive sources of
aluminum and template and exhibit difficulty to scale up.
2. Experimental
2.1. Synthesis procedure
* Corresponding author at: State Key Laboratory of Organic-Inorganic Compo-
sites, Beijing University of Chemical Technology, Beijing 100029, China.
Tel.: +86 10 82908332; fax: +86 10 64434784.
The reagents used were aluminum nitrate, ammonium
carbonate and PEG 1540. All of the chemicals used in this work
E-mail address: guof@mail.buct.edu.cn (F. Guo).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.10.023

Z. Tang et al. / Materials Research Bulletin 48 (2013) 290–294
291
Table 1
were AR grade. The experimental setup is illustrated in Fig. 1. It
Specifications of the RPB used in this study.
consisted of an ammonium carbonate feeding system, an alumi-
num nitrate and PEG 1540 mixture circulating system and an RPB.
The specifications of the RPB employed in this work are given in
Table 1. The packing consisted of stainless wire mesh purchased
from Beijing Hongyahong Mesh Sale Center, Beijing, China. Liquid
was jetted onto the inner edge of the rotor via a liquid distributor,
which has a circular titanium alloy nozzle with a diameter of
0.003 m to ensure a certain initial speed of the liquid flow.
The experimental procedure was as follows: after 11 g of PEG
1540 was dissolved into 2200 ml of 0.6 M aluminum nitrate
solution, the as-prepared solution was poured into the circulating
tank, and then forced to circulate between the RPB and the
circulating tank by a centrifugal pump at 0.2 m3/h. 1800 ml of
0.6 M ammonium carbonate solution was slowly introduced into
the RPB by a precision pump. The aluminum nitrate solution and
the ammonium carbonate solution flowed co-currently through
the packing of the RPB and reacted to produce a gel-like mixture. If
necessary, aqueous ammonia was added into the mixture to reach
Item
Unit
Value
Inner radius of RPB casing, Rc
Inner radius of the packing, ri
Outer radius of the packing, ro
Axial length of the packing, Z
Volume of the rotor, V
m
0.10
m
0.025
0.075
0.05
7.85 Â 10À4
0.00023
3.93 Â 10À5
871
m
m
m3
m
Diameter of stainless wire mesh, D
Volume of the packing, V’
m3
m2/m3
Surface area of the dry packing per
unit volume of the rotor, a
Voidage of the dry packing,
e
m3/m3
0.95
wide angle scans were performed at a 2
validate the amorphous state of the alumina.
The morphology of the samples was observed on a JEOL 4000
electron microscope operated at 200 kV. The samples were
ultrasonically dispersed in ethanol and then dropped onto the
carbon-coated copper grids prior to the observation.
u
range of 10–908 to
or maintain
a pH of 7.6. After ammonium carbonate was
consumed, the mixture further circulated between the RPB and
the circulating tank for a certain time (varying from 0 to 10 min,
called subsequent mixing time) before discharged into a beaker
and aged at 293 K for 6 h and then filtered to collect the precipitate.
The precipitate was washed by distilled water and dried at 353 K
for 12 h, followed by calcinating at 573 K in nitrogen for 3 h, 703 K
in nitrogen for 2 h, and 823 K in oxygen for 3 h. The temperature
ramp rate between all stages was 2 K/min.
3. Results and discussion
Table 2 shows the specific surface area (SA), total pore volume
(PV) and pore diameter (PD) of OMA synthesized under different
rotating speed, ammonium carbonate solution flow rate, alumi-
num nitrate solution flow rate and mixing time. It is found that SA
value was in the range of 191–277 m2/g, PV in the range of 0.21–
0.31 cm3/g, and particle size (PS) in the range of 3.1–4.0 nm.
The transmission electron microscope (TEM) image of OMA
sample R0330 is shown in Fig. 2. The as-prepared OMA exhibited
wormhole channel motifs while it was reasonably uniform in pore
size distribution. The XRD pattern of the OMA is shown in Fig. 3,
which confirms that the mesoporous alumina was amorphous.
Previous studies revealed that the high-gravity environment in
an RPB provides a significant amount of mechanical energy for the
suspension and greatly intensifies the efficiency of micromixing
and dispersion, which are the crucial conditions for the synthesis of
mesoporous alumina. Further studies suggested that the size and
dispersion of nano-sized inorganic precursor is the key factor to
form the organized mesoporous structure [17,18]. Thus, the
synthesis was performed at different high-gravity levels in order
to understand the formation mechanism of OMA.
2.2. Characterization
Nitrogen adsorption–desorption isotherms were measured at
77 K on a Micromeritics ASAP 2010 analyzer. The total spore
volumes (PV) and pore size distributions of the samples were
calculated by the Barrett–Joyner–Helenda (BJH) equation, and
surface area (SA) was calculated by the Brunauer–Emmett–Teller
(BET) equation.
The structure of the samples was analyzed by powder X-ray
diffraction (XRD) on a Siemens D500 diffractometer using Cu K
a
radiation source (l = 0.154 nm). Low angle diffraction with a 2u
range of 0.5–108 was used to investigate the long range order, and
By adjusting the level of high-gravity, a series of mesoporous
alumina samples were obtained. The nitrogen adsorption–desorp-
tion isotherms of the samples are shown in Fig. 4, and the
corresponding pore size distributions are shown in Fig. 5. It can be
seen that the as-synthesized OMA exhibited a narrower PD
distribution and a smaller PV at a higher gravity level of 1850 m/s2
Table 2
Properties of OMA synthesized at different conditions.
Sample Different condition
Factor
SA/m2 gÀ1 PV/cm3 gÀ1 D/nm
Value
R0327
R0328
R0514
R0327
R0330
R0907
R0330
R0912
R6171
R6172
R6173
Rotating speed (rpm)
1500
900
226
216
201
226
0.21
0.22
0.30
0.21
0.24
0.29
0.24
0.26
0.29
0.31
0.31
3.1
3.1
4.0
3.1
3.1
3.9
3.1
3.6
3.2
3.3
3.3
300
Ammonium carbonate 10
flow rate (ml/min)
30; 10a 243
30
0.26
0.42
0
191
243
198
270
277
277
Aluminum nitrate
flow rate (m3/h)
Subsequent mixing
time (min)
5
10
Fig. 1. Experimental setup (1) RPB; (2) motor; (3) liquid distributor; (4) liquid flow
meter; (5) centrifugal pump; (6) electrode; (7) pH meter; (8) circulating tank; (9)
frequency modulator; (10) precision pump; (11) storage tank.
a
The flow rates of ammonium carbonate solution were 30 ml/min for 40 min and
then 10 ml/min until ammonium carbonate was totally consumed.

292
Z. Tang et al. / Materials Research Bulletin 48 (2013) 290–294
0.30
1850 m/s2
665 m/s2
74 m/s2
0.25
0.20
0.15
0.10
0.05
0.00
2
3
4
5
6
7
8
9
10
Pore diameter (nm)
Fig. 5. The pore size distribution of OMA synthesized in the RPB.
Fig. 2. TEM image of OMA sample R0330.
9.0
10 ml/min
30; 10 ml/min
30 ml/min
7.5
6.0
4.5
3.0
1.5
0.0
1000
900
800
700
600
500
400
300
200
100
0
900
600
300
0
0
3
6
9
/º
0.0
0.2
0.4
0.6
0.8
1.0
0
20
40
60
80
100
Relative Pressure, p/p0
(º)
Fig. 3. XRD pattern of OMA sample R0330.
Fig. 6. The nitrogen adsorption–desorption isotherms of OMA synthesized at
different flow rate of ammonium carbonate solution.
9.0
7.5
6.0
4.5
3.0
1.5
0.0
1850 m/s2
665 m/s2
74 m/s2
tended to become smaller and more uniform with the increase of
the high-gravity level. Thus the narrow pore channel was formed,
leading to the formation of the mesopores after further treatment.
It can be concluded that the high-gravity level has great effects on
the pore size distribution, surface area, and mesoporous structure.
0.25
10 ml/min
30; 10 ml/min
30 ml/min
0.20
0.15
0.10
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, p/p0
Fig. 4. The nitrogen adsorption–desorption isotherms of OMA synthesized in the
RPB.
and 665 m/s2. The results can be explained in terms of the pore
structure pileup mechanism and the RPB features. Liquid was
dispersed into tiny drops, filaments and films in the packing of the
RPB, resulting in an excellent micromixing effect. Therefore, the
inorganic precursor particles, a main constituent of OMA pore wall,
2
3
4
5
6
7
8
9
10
Pore Diameter (nm)
Fig. 7. The pore size distribution of OMA synthesized at different flow rate of
ammonium carbonate solution.

Z. Tang et al. / Materials Research Bulletin 48 (2013) 290–294
293
0.42
8
7
6
5
4
3
2
1
0 min
5 min
10 min
0.36
0.30
0.24
0.18
R0330
R0912
0.12
0.06
0.00
0.0
0.2
0.4
0.6
0.8
1.0
2
3
4
5
6
7
8
9
10
Relative Pressure, p/p0
Pore Diameter (nm)
Fig. 8. The nitrogen adsorption–desorption isotherms of OMA synthesized at
Fig. 11. The pore size distribution of OMA synthesized with different mixing time.
different flow rate of aluminum nitrate solution.
0.40
It can be found from Figs. 6 and 7 that the flow rate of
ammonium carbonate solution had a significant influences on
OMA properties. The OMA synthesis process can be divided into
two stages: the nucleation stage and the figuration stage. Figs. 6
and 7 show that the mesoporous structure of OMA was improved
when the flow rates of ammonium carbonate solution were
30 ml/min (40 min) and 10 ml/min (60 min) in comparison with
10 ml/min. But when the flow rate of ammonium carbonate
solution maintained 30 ml/min in the entire synthesis process, the
OMA structure changed significantly. It reveals that the figuration
stage in OMA synthesis is the key stage for the formation of
mesopores, and the nucleation stage is helpful to improving the
pore structure.
0.35
R0330
R0912
0.30
0.25
0.20
0.15
0.10
0.05
0.00
The flow rate of aluminum nitrate solution also had
a
pronounced effect on OMA properties. A higher liquid flow rate
induces a higher injecting velocity, which could enhance the
micromixing efficiency of the liquid in the RPB. Figs. 8 and 9 show
that increasing the flow rate of aluminum nitrate solution could
enlarge the pore diameter and narrow the pore size distribution,
and the surface area of OMA decreased with the increase of the
pore volume and diameter, It is deduced that the similar residence
time of the inorganic precursor in the RPB is the key factor in the
formation of ordered mesopores.
2
3
4
5
6
7
8
9
10
Pore Diameter (nm)
Fig. 9. The pore size distribution of OMA synthesized at different flow rate of
aluminum nitrate solution.
Besides the factors discussed above, the subsequent mixing
time after the pH value was adjusted to 7.6 was also very important
to the OMA structure. The effects of mixing time on OMA
properties were investigated in the presence of the pore-
expanding agent, and the results are shown in Figs. 10 and 11.
Fig. 10 shows that the nitrogen amount adsorbed increased slightly
when subsequent mixing time increased from 0 to 5 or 10 min,
while Fig. 11 demonstrates that the effect of subsequent mixing
time on the pore size distribution can be ignored. Therefore, it can
be deduced that the mechanical stirring has little effect on the
OMA microstructure, indicating a high stability of such micro-
structure.
9.0
7.5
6.0
4.5
0 min
5 min
10 min
3.0
4. Conclusions
1.5
OMA was synthesized in an RPB with a reaction between
aluminum nitrate and ammonium carbonate in this work. It was
found that the level of high-gravity, liquid flow rate and the mixing
time had great effect on the mesoporous structure, and the structure
changed with the variation of the operation conditions. It was
deduced that the excellent micromixing capability of the RPB is
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure, p/p0
Fig. 10. The nitrogen adsorption–desorption isotherms of OMA synthesized with
different mixing time.

294
Z. Tang et al. / Materials Research Bulletin 48 (2013) 290–294
ˇ
ˇ
[6] C. Ma´rquez-Alvareza, N. Zilkova´, J. Pe´rez-Pariente, J. Cejka, Cat. Rev.: Sci. Eng. 50
helpful to the formation of OMA with narrow pore size distribution
and uniform structure, and the formation process of OMA involves
the nucleation stage and the figuration stage. This work may provide
a novel pathway for the synthesis of quality OMA.
(2) (2008) 222.
[7] Q.L. Wu, F. Zhang, J.P. Yang, Q. Li, B. Tu, D.Y. Zhao, Microporous Mesoporous Mater.
143 (2011) 406.
[8] W. Deng, P. Bodart, M. Pruski, B.H. Shanks, Microporous Mesoporous Mater. 52
(2002) 169.
ˇ
ˇ
ˇ
´
´
[9] N. Zilkova, N. Zilkova, A. Zukal, J. Cejka, Microporous and Mesoporous Mater. 95
Acknowledgement
(2006) 176.
[10] K. Niesz, P. Yang, G.A. Somorjai, Chem. Commun. (2005) 1986.
[11] V.V. Vinogradov, A.V. Agafonov, A.V. Vinogradov, K.T. Pillai, Rajesh V. Pai, S.K.
Mukerjee, S.K. Aggarwal, J. Sol–Gel Sci. Technol. 60 (2011) 6.
[12] Pasquale F. Fulvio, Reno I. Brosey, Mietek Jaroniec, Appl. Mater. Interface 2 (2010)
588.
This research was supported by the National Natural Science
Foundation of China (No. 20676010).
[13] C.C. Lin, T.J. Ho, W.T. Liu, J. Chem. Eng. Jpn. 35 (2002) 1298.
[14] Y.S. Chen, H.S. Liu, Ind. Eng. Chem. Res. 41 (2002) 1583.
[15] S. Bhattacharya, A. Mondal, A. Bhowal, S. Datta, Ind. Eng. Chem. Res. 49 (2010)
847.
[16] Y.H. Chen, C.Y. Chiu, C.Y. Chang, Y.H. Huang, Y.H. Yu, P.C. Chiang, J.L. Shie, C.S.
Chiou, Ind. Eng. Chem. Res. 44 (2005) 21.
[17] B.C. Sun, X.M. Wang, J.M. Chen, G.W. Chu, J.F. Chen, L. Shao, Chem. Eng. J. 168
(2011) 731.
[18] J.F. Chen, L. Shao, China Particuol. 1 (2003) 64.
[19] R.H. Zhao, C.P. Li, F. Guo, J.F. Chen, Ind. Eng. Chem. Res. 46 (2007) 3317.
References
[1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992)
710.
[2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, T.W. Chu,
D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem.
Soc. 114 (1992) 10834.
[3] A. Taguchi, F. Schuth, Microporous Mesoporous Mater. 77 (2005) 1.
[4] C. Misra, Industrial Alumina Chemicals, ACS Monograph 184, American Chemical
Society, Washington, 1986.
[5] C. Chen, W.S. Ahn, Chem. Eng. J. 166 (2011) 646.