Layered Silicates by Swelling of AMH‐3 and Nanocomposite Membranes

Abstract

Layered materials with nanoporous layers (e.g., , [1][2][3][4] AMH-3 [5,6] ) have structures intermediate between those of crystalline nanoporous frameworks, such as zeolites, and typical layered materials, such as clay minerals. Each layer includes a porous network, while the gallery between layers provides the ability for intercalation, pillaring, and exfoliation. [3][4][5][7][8][9][10][11][12][13] One such material, AMH-3, is the first layered silicate with internal porosity accessible from all directions through eight-membered ring (8 MR) apertures (i.e. pore openings made of eight SiO 4 tetrahedra). [5,6] It has been proposed to use exfoliated silicate layers of AMH-3 as a selectivity-enhancing additive in polymers. [14,15] A recent simulation study provides further motivation for the fabrication of nanocomposites incorporating dispersed AMH-3 layers for gas separation membranes.[16] However, fabrication of nanocomposites has not been demonstrated. Herein, we report on AMH-3 swelling using a novel procedure; the swollen material is used to prepare polymer nanocomposite membranes with improved selectivity.Typically, layered materials can be swollen by the intercalation of organic surfactants, such as quaternary alkyl ammonium ions or amine molecules, by cation exchange or hydrogen-bonding interaction with intergallery moieties. [3,4,[17][18][19][20][21][22][23][24] The swollen derivative of AMH-3 was prepared by intercalation of primary amine molecules (dodecylamine) after proton exchange in the presence of amino acid. In this procedure, an aqueous solution of dl-histidine was employed as a buffer and a source of protons to exchange the strontium and sodium cations in the original structure. The initial pH value was adjusted to be approximately 6.0 by addition of hydrochloric acid, and ion exchange was allowed to proceed at room temperature until the pH value reached approximately 6.4. At this point, the aqueous solution of dodecylamine was added. The swollen AMH-3 was obtained after twelve hours at 60 8C. As will be described in detail elsewhere, several alternative procedures failed to yield swollen AMH-3. A brief account of these attempts is shown in Figure 1.The emergence of a swollen material was monitored by various characterization techniques. ICP (inductively coupled plasma) chemical analysis shows that Na and Sr cations of the original structure (4.8 wt % Na, 20.3 wt % Sr) were exchanged, leaving almost no Na (0.7 wt %) and a smaller Figure 1. AMH-3 structure projection along the b axis showing silicon atoms (Si1, Si2, Si3, and Si4, light sticks), oxygen atoms (dark sticks), sodium cations (Na, single balls), and strontium cations (Sr, double balls). Details on the structure can be found in reference [5]. The arrows indicate attempted ion-exchange and swelling procedures. Only the last procedure, combination of ion exchange in the presence of dlhistidine and swelling using dodecylamine, resulted in swollen AMH-3. CTAB = cetyltrimethylammonium bromide, DTAB = dodecyltrimethylammonium bromide, DOA = do...

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“…The geometry of these fillers is not practically affected by the orientation within the polymer phase but limits the formation of thin selective layers [34]. Recent developments have used high aspect ratio (> 10) sieving materials (usually known as layered materials, fakes or platelets) as alternative fillers to traditional sieves in hybrid membranes [24,[35][36][37]. The features of layered materials such as high aspect ratio and small thickness can improve the separation performance by adding only a small volume fraction of these fillers within the MMM.…”

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