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MCM-41 139

6.4. MCM-41
Beck et al. (1992) succeeded in the synthesis of a new family of ordered, meso-
porous silicate/aluminosilicate by hydrothermal formation of silica gels in the
presence of surfactant templates. Quaternary ammonium surfactants were typ-
ically used. The surfactants self-assemble to form micellar templates with a
three-dimensional, long-range order. The silicate precursors condense on the walls
of the template. The organic templates are subsequently removed by air oxida-
tion leaving behind a silicate structure. These materials are amorphous but exhibit
◦
simple X-ray diffraction patterns (2θ ∼ 2 ) that reflect the interplanar spacing of
the regular mesoporous structure of the templates. These materials were named
M41S, and the materials that have the honeycomb-shaped structures are named
MCM-41. A schematic of the formation of MCM-41 is shown in Figure 6.6.
Different mechanisms for the interactions between the silicate precursors and
the organic template, as well as the liquid crystal templating mechanisms, have
been discussed (Ying et al., 1999; Tanev and Pinnavaia, 1995). Two other major
types of M41S materials are MCM-48 (with 3-D pores) and MCM-50 (with a
pillared layer or lamellar structure).
Among the M41S materials, MCM-41 has received the most attention because
of its simple structure, as well as the ease of synthesis and tailoring of its struc-
ture and surface properties. The pore dimension is in the range of 20–100 ˚ A
and can be tailored by several different strategies (Rouquerol et al., 1999; Ying
et al., 1999; Zhao et al., 1996). The first is to use surfactants with different
chain lengths and pore diameters that can be controlled from near 15 to 45 ˚ A
(Huo, et al., 1996). The use of two surfactants can extend the pore sizes to 55 ˚ A
(Kaman et al., 1996). The addition of expander molecules, such as trimethylben-
zene (Beck et al., 1992; Zhao et al., 1996), extends the pore size to near 100 ˚ A.
Good-quality, large-pore MCM-41 with pore sizes up to 65 ˚ A can be made by
post-synthesis hydrothermal restructuring (Huo et al., 1996; Sayari et al., 1997).
Ordered, nanostructured materials with even larger pores (up to 300 ˚ A) can be
obtained by using block copolymers as the structure-directing agents (Zhao et al.,
1998). Moreover, MCM-41 with heteroatoms (such as Al, Ti, B, and V) and many

(a) (b) (c)
Figure 6.6. Schematic representation of the formation of MCM-41 by the liquid-crystal templat-
ing mechanism. (A) Hexagonal array of cylindrical micelles; (B) the same, with silicate species
between the cylinders; and (C) hollow cylinders of MCM-41 after thermal elimination of organic
material (Rouquerol et al., 1999, with permission).