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376 Mesoporous Materials, Synthesis and Properties

be done before or after template removal. If it is done after consistofhighly disordered wormhole-likearrays ofchan-
template removal, the guest element is anchored onto the nels, and KIT, with a highly branched network of pores
silica framework via hydroxy (silanol) groups present on similar to the L 3 surfactant phase.
the surface. A third method is deposition of guest elements
(e.g., metals) onto the silica mesoporous structure via the
so-called incipient wetness impregnation technique. The V. PORE SIZE CONTROL
most common modified mesoporous silica materials are
those containing aluminum. Incorporation of transition One of the most attractive properties of mesoporous
metals into the silica structure is also fairly common. molecular sieves is the ability to control/tailor their pore
Indeed,thecompositionalflexibilityofmesoporousma- size over a wide range. Pore size in mesoporous materials
terials is such that it is possible to prepare mesoporous can be controlled by the length of the surfactant, addition
organosilicas with organic groups (e.g., ethene) inside of auxiliary organics, the template removal method, and
the pore walls. An unusual property of mesoporous sil- processing conditions.
icas is that they can themselves be used as templates
for the formation of mesoporous carbon. The mesoscopi-
A. Surfactant Chain Length
cally ordered nanoporous (or mesoporous) carbon molec-
ular sieves are prepared by carbonizing sucrose (or other Some limited control of pore size is possible simply by
carbon precursors) inside the pores of mesoporous silica changing the length of the surfactant chain. A good ex-
molecular sieve. The mesoporous carbon is obtained after ample is for trimethylammonium bromide (CnTMABr,
subsequent removal of the silica template by dissolution n = 8, 10, 12, 14, 16, 18) surfactants, where the pore size
˚
in HF (hydrofluoric acid) or NaOH (sodium hydroxide) increases by roughly 2.25 A for each increase of one car-
solution. bon in the surfactant. This means that even-numbered car-
bon chain length surfactants can be used to increase the
˚
pore size by increments of ∼4.5 A. A limitation of this
B. Morphology
method of pore size control is that the shortest chain sur-
Mesoporous materials can be made in a variety of forms. factant from which mesophases can be made is n = 8 (im-
˚
These include bulk powders (the most common form), posing a lower limit of pore size of ∼15 A). An upper
˚
monolithic gels, and thin films, hierarchically ordered fi- limit of possible pore size is ∼45 A due to the limited sol-
brous/tubular forms, and “hard” spheres. The form ob- ubility of surfactants with n > ∼18 carbons which cannot
tained depends on the synthesis conditions and in partic- therefore be used.
ular the rate of precipitation of the inorganic species. For
silica materials, low pH values are used to slow down the
B. Adding Auxiliary Organics
rate of silicate precipitation, thus making it easier to con-
trol the morphology and form. Mesophases can be made with regular pore diameters up
˚
to ∼80 A by dissolving hydrophobic molecules into the
interior of the surfactant aggregates prior to extensive con-
IV. PORE GEOMETRY densation of the inorganic framework. Auxiliary organics
that can be used include paraffins, aromatics, and alcohols.
The surfactant/inorganic interactions determine the pore For example, 1,3,5-trimethylbenzene (TMB) provides a
geometry of the resulting mesoporous material. For ex- nearly linear increase in pore size with increasing concen-
ample, the M41S family contains three distinctly different tration up to a TMB/Si ratio of 2.5.
mesophases; a hexagonal (space group p6m) phase re-
ferred to as MCM-41, a cubic phase (space group Ia3d)
C. Hydrothermal Restructuring
known as MCM-48, and a nonstable lamellar phase (space
group p2). MCM-41 possesses nonintersecting (one- Surfactant-containing silica and aluminosilicate meso-
dimensional) pores while MCM-48 has a bicontinuous phasescanbehydrothermallyrestructuredatelevatedtem-
◦
three-dimensional pore structure (see Fig. 1). Other peratures (100–175 C) in alkaline solutions resulting in
mesophases, which illustrate the breath of inorganic/ pore size expansion.
surfactant interactions are SBA-1, cubic phase (space
group Pm3n), and SBA-2, hexagonal phase (space group
D. Processing Conditions
P6 3 /mmc) with supercages instead of unidimensional
channels. In addition, other less well-ordered phases have Reaction (synthesis) time, degree of framework conden-
been observed. These include HMS and MSU-n, which sation, or template removal conditions can affect the pore