Synthesis of Mesoporous Silica Materials by Using Anionic Surfactants as Template

Abstract

1.Introduction 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 1990s 1),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because of emerging applications in catalysis, adsorption, sensors, and separation 3) .Meso-porous materials have superseded zeolite molecular sieves, which have pore size restricted to around 1.5 nm.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 meso-porous silicas (M41S family and SBA-1, 2, 3, 7, etc.) are prepared using cationic quaternary ammonium sur-factants and based on electrostatic interaction S + I − or S + X − I + (Iinorganic precursor) pathways 4)8) .The hydrogen-bonding interactions S 0 I 0 (HMS family) 9)17) and (S 0 H +)(X − I 0) (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 meso-structured composite.The mesoporous materials are obtained by subsequent removal of the surfactant by extraction or calcination.Many well-ordered meso-porous materials have been successfully synthesized with diverse mesostructures such as orthorhombic (Pmmm 21) etc.), tetragonal (P42/mnm 22) , P4/mmm 21)), three-dimensional (3D) hexagonal (P63/mmc 23)), micel-lar cubic (Pm3 ¯ n 7),24) , Fd3 ¯ m 25),26) , Im3 ¯ m 19),24) , Fm3 ¯ m 27) , Pm3 ¯ m 19) , etc.), bicontinuous cubic (Ia3 ¯ d 2),28) d 2),28) d , Im3 ¯ m 29) and Pn3 ¯ m 30)), rectangular (c2mm 31)), two-dimensional (2D) hexagonal (p (2D) hexagonal (p (2D) hexagonal (6mm 2),18)), and lamellar phases 2). A wide range of surfactants are commonly used in 299 Mesoporous materials have attracted a great deal of attention because of their controllable structures and compositions, which make them suitable for a wide range of applications in catalysis, environmental clean-up, and development of advanced materials.Various mesoporous materials can be synthesized based on the self-assembly of surfactants and inorganic precursors.We previously demonstrated a novel templating route for preparing mesoporous silicas based on the self-assembly of anionic surfactants and inorganic precursors in the presence of aminosilane or quaternized aminosilane as a co-structure-directing agent (CSDA).We continue to investigate the resultant novel mesoporous materials called anionic surfactant mesoporous silica (AMS).The latest AMS series is a novel bicontinuous cubic Pn3 ¯ m mesoporous silica (AMS-10).We have also found that the as-synthesized AMS is transformed into amino-functionalized mesoporous silica, by removal of only the surfactant by extraction, with potential uses as high-performance catalysts and adsorbents.Furthermore, the use of a chiral anionic surfactant derived from the amino acid, N-myristoyl-N-myristoyl-N l-alanine sodium salt, provides chiral mesoporous silica.Such anionic surfactant templating routes will provide a new family of mesoporous materials as well as information on the structural behavior of anionic surfactants.This review describes the preparation, properties, and potential applications of anionic surfactant templated mesoporous silicas.