Synthesis of Mesoporous Silica Materials by Using Anionic Surfactants as Template

1. 1. Well-ordered Mesoporous Materials FSM-16 and M41S materials with regularly ordered mesopore arrangements and narrow pore-size distributions were fi rst described in the early 1990s1),2), and the mesostructures of silica and various metal oxides have since attracted much attention from chemists and materials scientists because of emerging applications in catalysis, adsorption, sensors, and separation3). Mesoporous materials have superseded zeolite molecular sieves, which have pore size restricted to around 1.5 nm. Like the microporous crystalline zeolites, mesoporous materials are characterized by very large specific surface areas, ordered pore systems, and uniformed-sized pores. Unlike the zeolites, mesoporous materials have large pore diameters ranging from approximately 2 to 50 nm and amorphous walls. 1. 2. Formation Mechanism The formation of mesostructured materials is achieved by a micelle-templating method, based on electrostatic charge-matching and electrically neutral pathways in the presence of surfactants (S) as a structure directing agent (SDA)4),5). The well-ordered mesoporous silicas (M41S family and SBA-1, 2, 3, 7, etc.) are prepared using cationic quaternary ammonium surfactants and based on electrostatic interaction S+I− or S+X−I+ (I=inorganic precursor) pathways4)~8). The hydrogen-bonding interactions S0I0 (HMS family)9)~17) and (S0H+)(X−I0) (SBA-11, 12, 15, 16 and FDU-1, 2, 5)18)~20) are operative in the preparation of mesoporous silicas with neutral nonionic surfactants. These SDAs, in the form of a lyotropic liquid-crystalline phase, lead to the assembly of the silica species during the condensation of the silica precursors to form an ordered mesostructured composite. The mesoporous materials are obtained by subsequent removal of the surfactant by extraction or calcination. Many well-ordered mesoporous materials have been successfully synthesized with diverse mesostructures such as orthorhombic (Pmmm21) etc.), tetragonal (P42/mnm, P4/mmm21)), three-dimensional (3D) hexagonal (P63/mmc), micellar cubic (Pm3̄n7),24), Fd3̄m25),26), Im3̄m19),24), Fm3̄m27), Pm3̄m19), etc.), bicontinuous cubic (Ia3̄d2),28), Im3̄m29) and Pn3̄m30)), rectangular (c2mm31)), two-dimensional (2D) hexagonal (p6mm2),18)), and lamellar phases2). A wide range of surfactants are commonly used in 299 Journal of the Japan Petroleum Institute, 50, (6), 299-311 (2007)