The synthesis of MCM-48 with high yields

Article abstract of DOI:10.1016/S0025-5408(01)00645-6

Using tetraethyl orthosilicate as a silica source and cetyltrimethylammonium bromide as a template, MCM-48 mesoporous molecular sieves were synthesized with hydrothermal methods. Reducing the pH of the solution during synthesis improved the yield of MCM-48 and enhanced its stability. While the pH was adjusted to 5, a high yield of 98% was gained. XRD and N₂ adsorption-desorption isotherms were used to characterize the synthesized samples. These mesoporous molecular sieves showed high specific surface areas and ordered channel systems.

Full text of DOI:10.1016/S0025-5408(01)00645-6

Materials Research Bulletin 36 (2001) 1717–1720
Pergamon
The synthesis of MCM-48 with high yields
Shuguo Wang, Dong Wu, Yuhan Sun*, Bing Zhong
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry Chinese Academy of Sciences,
Taiyuan 030001, P. R. China
(Refereed)
Received 22 November 2000; accepted 16 March 2001
Abstract
Using tetraethyl orthosilicate as a silica source and cetyltrimethylammonium bromide as a tem-
plate, MCM-48 mesoporous molecular sieves were synthesized with hydrothermal methods. Reducing
the pH of the solution during synthesis improved the yield of MCM-48 and enhanced its stability.
While the pH was adjusted to 5, a high yield of 98% was gained. XRD and N2 adsorption–desorption
isotherms were used to characterize the synthesized samples. These mesoporous molecular sieves
showed high specific surface areas and ordered channel systems. © 2001 Elsevier Science Ltd. All
rights reserved.
Keywords: A. Nanostructures; B. Chemical synthesis
1. Introduction
MCM-48 is a member of M41S mesoporous silicas. It has recently attracted much
attention for its three-dimensional mesoporous channel systems, which can be used as a
prospective catalyst, an adsorbent, and even a template for the synthesis of nanostructures.
A lot of work has been done in the synthesis of pure silica MCM-48 [1–4] and the
incorporation of its framework with various heteroatoms [5–7]. MCM-48 was usually
synthesized using cationic surfactants as supramolecular template materials. Cationic-neutral
or cationic-anionic surfactants were also used as its structure-directing agent [1,8]. With
almost no exception, MCM-48 was prepared in basic solution, and its yields were usually
low. The product yield was only ca. 50% using cetyltrimethylammonium bromide (CTAB)
as a single surfactant [9]. Ryoo gained a yield of ca. 80% using cationic-neutral surfactant
* Corresponding author. Fax: 86-351-4041153
E-mail address: yhsun@sxicc.ac.cn (Y. Sun).
0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S00025-5408(01)00645-6

1718
S. Wang et al. / Materials Research Bulletin 36 (2001) 1717–1720
Fig. 1. N2 adsorption–desorption isotherms and pore size distribution of the sample M2.
mixture and adjusting the pH of the reaction mixture to 10 during synthesis [3]. It is
important to identify procedures suitable for a cost-efficient synthesis of MCM-48 with high
yields. Herein, by adjusting the pH of the solution in a synthesis process, a high product yield
of 98% was gained using CTAB as a surfactant.
2. Experimental
Sodium hydroxide and CTAB were dissolved in deionized water, and tetraethyl ortho-
silicate (TEOS) was added into it. The molar composition of the gel was TEOS/0.48
NaOH/0.48 CTAB/55 H2O. The solution was stirred for about 1 h and then transferred into
a Teflon bottle and heated up to 373 K. After 3 days the solution was cooled to room
temperature and its pH was adjusted by adding HCl. Then it was heated at 373 K for 1 to 3
days. The product was filtered, washed, and dried in air at ambient temperature. The dried
product was finally calcined at 823 K for 6 h. The obtained samples were characterized by
XRD, which was performed using a D/max-rA X-Ray Diffractometer, and nitrogen adsorp-
tion, which was performed using ASAP2000M Accelerated Surface Area and Porosimetry at
77 K.
3. Results and discussion
When the pH of the solution was adjusted to 5–9 during synthesis, MCM-48 could be
synthesized. But amorphous silica was gained when the pH was adjusted to 3.
The N2 adsorption–desorption isotherm of a typical MCM-48 (the sample M2) synthe-
sized by pH adjustment is shown in Fig. 1, which gives a BET surface area of 921 m2/g and

S. Wang et al. / Materials Research Bulletin 36 (2001) 1717–1720
1719
Fig. 2. XRD pattersn of MCM-48 (H1-heated in boiling water for 8 h; H2 for 16 h).
shows a pore size of 2.6 nm (BJH model). A sharp increase in the volume of N2 adsorbed
is observed at a relative pressure of p/p0 ϭ 0.3–0.4. The XRD patterns in Fig. 2 indicate
excellent structure order for the products.
Adjusting the pH to a relatively low value drastically improved the yields of the synthe-
sized molecular sieves (see Table 1). A product yield as high as 98% was gained when the
pH was adjusted to 5; however, the yield of MCM-48 was not more than 60% before the pH
adjustment (pH ϭ 12), which was consistent with literature [10]. The stability of MCM-48
was also enhanced. After heating in boiling water for 8 h, the structure of the sample M0 was
damaged more apparently than that of M1, M2, and M3; for 16 h, the mesostructure of the
sample M0 collapsed thoroughly, but that of M1, M2, and M3 still remained (see Fig. 2).
Table 1
The synthesis conditions and characteristics of MCM-48
Sample
pH
adjusted
t**/day
Yield/%
d211/nm
Unit cell
a0/nm
As-synthesized
Calcined
M0
M1
M2
M3
M4
M5
M6
no
7
7
7
9
0
1
2
3
2
2
2
58
95
97
96
85
98
4.08
4.09
4.09
4.10
4.10
3.50
4.02
3.98
3.98
4.02
3.82
8.57
9.85
9.75
9.75
9.85
9.36
5
3
amorphous
t**: the time heated for after pH adjustment.

1720
S. Wang et al. / Materials Research Bulletin 36 (2001) 1717–1720
In the solution, anionic silicate species associated at the micelle interface and then
polymerized into extended framework that led to the formation of mesoporous structure.
When a part of anionic silicate species dissolved in strong basic solution, the yield of the
synthesized material was low. When the pH was reduced, the dissolved silicate would
aggregate, and might further condense with the hydroxyl on the wall surface, then the yield
of synthesized MCM-48 increased and its wall became thicker, so the unit cell shrunk less
during calcination, which could be seen in Table 1. MCM-48 became more stable because
of the thicker wall. When the pH was reduced to 3, in moderate acid solution, a part of
surfactant cations would go out of the channel of MCM-48 and the mesoporous structure was
destroyed because of its low hydrothermal stability, so amorphous silica was gained.
Acknowledgment
The financial support from National Science Foundation of China (29973057, 29625307)
and National Key Basic Research Special Foundation (G20000480) were gratefully acknowl-
edged.
References
[1] F. Chen, L. Huang, Q. Li, Chem. Mater. 9 (1997) 2685–2686.
[2] J. Xu, Z. Luan, H. He, W. Zhou, L. Kevan, Chem. Mater. 10 (1998) 3690–3698.
[3] R. Ryoo, S.H. Joo, J.M. Kim, J. Phys. Chem. B 103 (1999) 7435–7440.
[4] K. Schumacher, M. Gru¨n, K.K. Unger, Microporous Mesoporous Mater. 27 (1999) 201–206.
[5] A. Corma, Q. Kan, F. Rey, Chem. Commun. (1998) 579–580.
[6] Q. Cai, C. Wei, Y. Xu, W. Pang, K. Zhen, Chem. J. Chin. Univer. 20 (1999) 344–349.
[7] K. Schumacher, C.F. Hohenesche, K.K. Unger, R. Ulrich, A. Chesne, U. Wiesner et al., Adv. Mater. 11
(1999) 1194–1198.
[8] Y.-W Yan, H.-Y. Chen, Q.-Z. Li, Acta Chim. Sinica 56 (1998) 1214–1217.
[9] J.M. Kim, S.K. Kim, R. Ryoo, Chem. Commun. (1998) 259–260.