(256b) Swelling, Functionalization, and Structural Changes of the Nanoporous Layered Silicate AMH-3
The nanoporous silicate layered material AMH-3 contains nanometer-thick silicate sheets of crystalline porous structure. Layered materials of this type can be considered as intermediate between porous 3D zeolite frameworks and nonporous layered clay minerals. AMH-3 is the first 3D-nanoporous layered material. It has 8-membered-ring (8MR) pores like conventional zeolite frameworks within the individual layers, and it also has a stacked-layer structure similar to that of clay minerals. This research is expected to improve the performance of the current conventional zeolite / polymer composite membrane by using the thin and high aspect ratio inorganic nanolayers in polymer matrix. In comparison to conventional zeolites, layered materials such as AMH-3 have advantages such as significantly increased surface area and decreased diffusion limitations. These properties are particularly promising for applications as materials for enhancing selectivity and throughput in composite membranes. In such membranes, the layered material would ideally be exfoliated into individual layers (or stacks of a few layers) and dispersion uniformly thoughout a polymeric matrix.
An important issue in the fabrication of such membranes is the development of chemistries for swelling and exfoliation of AMH-3 and other layered materials, and the characterization (including structural changes) of these modified materials. In this work, we have firstly investigated the swelling and functionalization process of AMH-3 by using different swelling agents. In order to swell the AMH-3, two different kinds of swelling agents ? amines and diamines - were used. The XRD patterns and TEM images clearly show differences in the swollen AMH-3. For the amine-swollen AMH-3, the gallery space between the individual layers was 41Å, reflecting the construction of the bilayer stacks of the swelling agent. The gallery spacing of the diamine-swollen AMH-3 was 25 Å, indicating the single layer stacks. The swelling step also causes significant structural changes due to an intralayer condensation reaction. The tetrahedral inversion of the Q3 silicon makes the hydroxyl groups condense in the individual layers, thus the Q3 silicons are condensed to Q4 silicons. Solid state 29Si-NMR shows that the initial Q3 and Q4 ratio of 3:1 was changed to 1:3 and 1:1 for the amine and diamine-swollen AMH-3, respectively. The structure change is less severe in diamine-swollen AMH-3 and is more beneficial for the preservation of the pore structures, as it maintains the advantages of the 3-D nanoporous layered material to enhance the selectivity.
Secondly, the functionalization of AMH-3 was performed by condensation between surface hydroxyl group and the mono-, di-, tri-methoxysilanes. Chemical shifts (ppm) and Q3 and Q4 ratio changes depending on the different silanes will be discussed. Due to the formation of strong covalent bonds with the hydroxyl groups on the silicate layer, this functionalization method can prevent the intralayer condensation as it preserves the AMH-3 structure during the intercalation step. The hydrophobicity of the functionalized AMH-3 can also be controlled for incorporation into various kinds of polymer matrices. The application of swollen and functionalized AMH-3 to the formation of composite (mixed matrix) membranes will also be discussed.