Design of Molecularly Ordered Framework of Mesoporous Silica with Squared One-Dimensional Channels

Abstract

Mesoporous silica with squared one-dimensional channels (KSW-2-type mesoporous silica), possessing a molecularly ordered framework arising from a starting layered polysilicate kanemite, was obtained through silylation of a surfactant (hexadecyltrimethylammonium, C16TMA)-containing mesostructured precursor with octoxytrichlorosilane (C8H17OSiCl3) and octylmethyldichlorosilane (C8H17(CH3)SiCl2). The presence of the molecular ordering in the silicate framework was confirmed by XRD and TEM. Octoxy groups grafted on KSW-2 can be eliminated by subsequent hydrolysis under very mild condition, and pure mesoporous silica was obtained with the retention of the kanemite-based framework. The framework is structurally stabilized by the attachment of additional SiO4 units to the framework, and the mesostructural ordering hardly changed under the presence of water vapor. A large number of silanol groups remained at the mesopore surfaces because C16TMA ions and octoxy groups can be removed without calcination. Octylmethylsilyl groups are regularly arranged at the mesopore surface due to the molecular ordering in the silicate framework. The molecularly ordered structural periodicity originating from kanemite is retained even after calcination at 550 degrees C, while that in the precursor without silylation disappeared. The synthetic strategy is quite useful for the design of the silicate framework of mesostructured and mesoporous materials with and without surface functional organic groups.

Ordered Mesoporous Silica Derived from Layered Silicates

“…The retention of the structural units has recently been shown in more detail. [73] Both the mesostructure and the periodic silicate structure in as-synthesized KSW-2 were confirmed by XRD and TEM. In addition to regularly arranged 1D squared mesopores in the TEM images, slightly arc-shaped diffraction spots, corresponding to approximately 0.37 nm, can be detected successfully perpendicular to the spots of the mesochannel ordering in the selected-area electron diffraction (SAED) pattern.…”

“…A new synthetic strategy is proposed to design the periodic kanemite-based framework chemically by direct silylation of as-synthesized KSW-2 to stabilize the structure. [73,74] The details will be described in Section 6.…”

“…The kanemite-based periodic structure is stabilized structurally by direct organic modification of as-synthesized KSW-2 with suitable silylating agents such as octoxytrichlorosilane (C 8 H 17 OSiCl 3 ). [73,74] The fixed organic groups can be removed by hydrolysis under very mild conditions. Once the structural units are modified by direct silylation, the new framework is retained even after calcination.…”

“…XRD patterns of directly silylated and hydrolyzed KSW-2 with C 8 H 17 OSiCl 3 . Reproduced with permission from [73]. Copyright 2008, American Chemical Society.…”

“…Furthermore, the structural units in KSW-2-based mesoporous silica can be chemically designed and structurally stabilized by direct silylation of the mesostructured precursor, successfully leading to the formation of ordered mesoporous silica with a molecularly ordered framework, which will be considered crystalline. [73,74] Therefore, this Feature Article summarizes the research on ordered mesoporous silica derived from some layered silicates and also covers a large body of the research on their applications to discuss the uniqueness of the ordered mesoporous silica. This contributes to further understanding of the research on other types of mesoporous silica such as MCM-and SBA-type series, which are compared with the uniqueness and shortcomings of FSM-and KSW-type mesoporous silica.…”