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






               374                                                                   Mesoporous Materials, Synthesis and Properties








                                                                 FIGURE 6 Scheme showing the folding sheet mechanism for the
                                                                 formation of mesoporous materials.


                                                                 sponsible for the formation of a majority of mesoporous
                                                                 materials currently available. Key to this mechanism is
                                                                 the interaction between the surfactant molecules and inor-
                                                                 ganic species. Based on pathway 2 of the LCT mechanism,
                                                                 the interaction between anionic (negatively charged) inor-
                                                                 ganic species (I ) and cationic (positively charged) qua-
                                                                              −
                                                                 ternary ammonium surfactants (S ) is categorized as S I
                                                                                           +
                                                                                                            + −
                                                                 interaction. There are at least four different electrostatic
                                                                 reaction pathways/interactions that can be explained by a
                                                                 LCT type of mechanism.

                                                                 1. The (S I ) route: In this case, cationic surfactants
                                                                         + −
                                                                    (S ) are used as structure directors for anionic
                                                                      +
                                                                    inorganic species (I ). This is recognized as the
                                                                                    −
                                                                    pathway for the syntheses of M41S mesoporous
                                                                    materials.
                                                                 2. The (S I ) route: In this pathway anionic surfactants
                                                                         − +
                                                                      −
                                                                                                         +
               FIGURE  5  Scheme  showing  the  formation  of  the  silicatropic  (S ) interact with cationic inorganic species (I ).
                                                                            − +
                                                                         +
               liquid-crystal phase(s).                          3. The (S X I ) route: Here both the surfactant and the
                                                                    inorganic species are cationic. This pathway is
               molecular sieve synthesis where strong electrostatic inter-  mediated by negatively charged counterions
                                                                           −
                                                                      −
                                                                                −
               actions are absent—for example when neutral surfactants  (X = Cl ,Br etc).
                                                                            + −
                                                                         −
               are used. Neutral surfactants have a much greater tendency  4. The (S M I ) route: In this case both the surfactant
               to form aggregates in water than charged surfactants, and  and the inorganic species are anionic and positively
                                                                                       +
                                                                                             +
                                                                                                +
               their strong tendency to aggregate in combination with  charged counterions (M = Na ,K , etc.) mediate
               hydrogen-bonding interactions is apparently sufficient to  the structure formation.
               allow the formation of mesophases.
                                                                 This classification is very useful especially for other types
                                                                 of surfactant–inorganic interactions such as those involv-
                 2.  Folded Sheet Mechanism
                                                                 ing neutral surfactants as structure directors (templates).
               The second general formation mechanism for mesoporous  Such interactions can be denoted as follows:
               molecular sieves involves the intercalation of surfactants
                                                                     0 0
                                                                                               0
               into the layers of a layered silica, kanemite, followed by  1. S I , where both the surfactant (S ) and inorganic
                                                                            0
               conversion into a hexagonal phase structure. This mecha-  species (I ) are neutral. The surfactant–inorganic
               nism is therefore based on the intercalation of ammonium  interaction is due to hydrogen bonding.
                                                                     0 0
               surfactant in kanemite, a type of hydrated sodium polysil-  2. N I , where both the template, nonionic polyethylene
                                                                                                0
                                                                           0
               icate composed of single layered silica sheets. After the  oxide (N ) and inorganic species (I ) are neutral. The
               surfactants are ion exchanged into the layered structure,  organic–inorganic interaction is due to hydrogen
               thesilicatesheetsarethoughttofoldaroundthesurfactants  bonding.
               and condense into a hexagonal mesostructure as shown in
               Fig. 6.                                             The interactions listed above represent the majority
                                                                 of synthesis pathways to mesoporous molecular sieves.
                                                                 However, pathways involving other structure directing
               C. Generalized LCT Pathways
                                                                 agents, such as polymers, lyotropic liquid crystals, and
               As mentioned above, it is now generally accepted that  emulsions are also possible. Similarly, a wide range of
               pathway 2 of the LCT mechanism described above is re-  inorganic species may be used as discussed in Section III.
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