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Encyclopedia of Physical Science and Technology EN009I-420 July 10, 2001 15:8

370 Mesoporous Materials, Synthesis and Properties

Another common feature of mesoporous materials is that In recent years, fundamental and industrial demands
they are derived from assemblies of surfactants (or other have meant that considerable effort has been devoted,
similar aggregates), which act as templates for the organ- particularly in the last 20 years, to expanding the pore
isation of inorganic components into a structurally well- size range of well ordered zeolite-type (zeotype) materi-
defined framework. als into the mesoporous range. Using traditional zeolite
This article describes the preparation, properties (in- synthesis methods, the largest pore sizes achieved are in
cluding characterization techniques), and potential appli- the range 0.8–1.3 nm (in, for example, VPI-5, UTD-1,
cations of mesoporous materials. The article begins by and Cloverite). The development of mesoporous molecu-
examining the chemistry of surfactant/inorganic precur- lar sieves owes much to the considerable synthetic effort
sor solutions and discusses its application to the synthesis that, over the last two decades, has been devoted to devel-
of mesoporous materials. The most common methods of oping porous solids that bridge the gap between microp-
characterization and the properties of the materials are de- orous (e.g., zeolites) and macroporous (e.g., amorphous
scribed and finally potential applications are mentioned. aluminosilicates) materials. The hope was that new meso-
This article is therefore intended to provide a general porous materials would overcome the limitations imposed
overview of the synthesis, formation mechanisms, char- by the size of microporous zeolite pore channels. One such
acterization, properties, and applications of mesoporous group of materials, developed early on in the search for
molecular sieves. mesoporous solids, are pillared clays. Pillared clays are
prepared by the “propping” apart of layered clay minerals
(usually of the smectite type, e.g., montmorillonite) with
I. INTRODUCTION AND BACKGROUND a variety of nano-sized pillars. The pillars are normally
metal oxides, which hold apart the clay layers and expose
Materials commonly referred to as molecular sieves ob- intracrystal clay surfaces. This increases the surface area
tained their name from the observation that they only ad- of the clay. The pore channels in pillared clays are de-
sorb molecules that are small enough to fit through their termined by the interlayer and interpillar spaces. Pillared
pore channel apertures. The implication of this is that clays have been shown to act as molecular sieves, adsor-
they can be used to separate molecules of different sizes. bents, and catalysts. However, they are not truly meso-
According to IUPAC (International Union of Pure and porous since they contain a heterogeneous mix of pore
Applied Chemistry) definitions, molecular sieves are de- channel sizes ranging from the microporous to the meso-
fined as one of three different types, depending on the porous range. A breakthrough for increasing pore sizes
size of the pores they possess—microporous (<2 nm), into the mesoporous range came in the early 1990s when
mesoporous (2–50 nm), and macroporous (>5 nm). Mi- truly mesoporous molecular sieves were synthesized by
croporous molecular sieves, such as zeolites, have been researchers in the United States (at the Mobil Corporation)
known for a long time. Indeed, zeolites were first reported and in Japan. Since then there has been a global resurgence
in 1756 by the Swedish geologist, Cronstedt, when he no- in interest in mesostructured materials that have uniform
ticed that on heating an unidentified silicate mineral in pore channels whose size is in the mesoporous domain.
a blowpipe flame, it appeared to boil—the word zeolite Such mesostructured materials are of great interest for a
is derived from the Greek words zeo—boil—and litho— number of reasons. They are unique in that they possess
stone. Today, the basic synthetic procedure for produc- extremely large uniform pores whose size can be easily
ing microporous molecular sieves (zeolites) is relatively tailored in a wide pore size range.
straightforward and is generally carried out in a high- The formation of mesoporous molecular sieves is es-
pressure autoclave at elevated temperatures. The inorganic sentially due to the use of surfactant molecular arrays (so-
precursor—for example, colloidal silica (and alumina if called supramolecular aggregates) as template rather than
the zeolite is to be an aluminosilicate)—is mixed with a the single solvated organic molecules or cations tradition-
specific organic molecule or a large cation (the template) ally used for the synthesis of microporous zeolites. It is
and heated to between 100 and 200 C. This process allows worth noting that the extent to which actual templating oc-
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the inorganic precursor to form into an open structure, sur- curs varies in zeolites and mesoporous molecular sieves.
rounding the suspended molecular templates. Finally, the In zeolites, the organic molecule rarely acts as a true tem-
solid porous zeolite product is obtained by burning away plate but typically directs structure or fills space in the
the template molecule (at ca. 500 C in air). The resulting porous product. The single organic template molecules
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zeolite has a highly regular and precise network of chan- are able to rapidly rotate in solution and therefore their
nels (micropores) whose size (and nature) can be varied by orientation is not fixed. Furthermore, the atomic sizes of
changing the template, inorganic precursor or preparation the organic molecule, which are comparable to the size
conditions. of the framework building units, means that only a rather