Synthesis, Characterization, and Application of Microporous Layered Silicate AMH-3 with Selective Ion Exchange and Transport Properties

Abstract

Microporous materials have been attracting great interest because of their ability to interact with atoms, ions and molecules not only at their surfaces, but throughout the bulk of the material. The utility of microporous materials is manifested in their microstructures (pore size, distribution, shape, volume), which allow molecules access to large internal surfaces and cavities that enhance catalytic activity and adsorptive capacity. Owing to this structures, microporous materials are widely used in various applications such as catalysts, adsorbents, molercular sieves. AMH-3 is a layered microporous material constructed from silicate layers and interlayer spaces occupied by strontium cations, sodium cations, and water molecules. AMH-3 is the first layered silicate with micoroporosity through eight-membered ring (8-MR) apertures along the thickness of the silicate layer as well as in the plane of the layers. Because of this unique structure, AMH-3 has three-dimensional ordered microporosity and good acid, chemical and thermal stability like zeolite and provides the ability for intercalation, pillaring, and exfoliation like layered silicate. Synthesis and characterization of AMH-3 and its synthesis mechanism study were conducted. Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), 29Si magic angle spinning nuclear magnetic resonance (MAS NMR), fourier transform infrared (FT-IR), and Raman spectroscopy were used to characterize AMH-3. A reasonable model for the synthesis mechanism of AMH-3 is proposed on the basis of experiment results. In the synthesis process, strontium and titanium act as structuring agent and assembly agent, respectively. The ion exchange behavior of AMH-3 and the removal of heavy metals present in aqueous solutions by AMH-3 are investigated for the first time. Pristine AMH-3 and ion-exchanged AMH-3 were characterized with ICP-AES, FE-SEM, 29Si CP MAS NMR and XRD. The studies on ion exchange behavior reveal that the sorption of ion by AMH-3 was found to be governed by ion exchange rather than surface adsorption, and no significant change occurred in the structure of the AMH-3 during the ion exchange. The removal of various heavy metal ions (Pb2+, Cu2+, Cd2+, and Zn2+) onto AMH-3 from aqueous solutions was conducted using a batch method. The effects of influential parameters, such as the initial metal ion concentration and contact time, on the sorption process were studied. The heavy metal ion sorption capacity and removal efficiency were mainly dependent on the difference between the effective pore size of the AMH-3 and the hydrated radius of the metal ion. The sorption isotherm data were well fitted by Langmuir (for Pb2+, Cu2+, and Zn2+) and Freundlich (for Cd2+) models. The sorption kinetics data were well fitted by a pseudo-second order kinetic model. Competitive sorption experiments revealed an order of metal ion affinity of Pb2+ > Cu2+ > Zn2+ > Cd2+. These findings indicate that AMH-3 is suitable for the efficient and selective removal of heavy metals from aqueous solutions. In order to use as permselective barrier, the removal of the Na+ and Sr2+ cations located in the intralayer and interlayer spaces for activation of micropores and the delamination of an individual layer must be carried out. We successfully obtained delaminated AMH-3 by acid-hydrothermal treatment without the use of swelling agent. The post-treatment conditions have been optimized to find the best morphology for use as permselective barrier. The pristine AMH-3 and the delaminated AMH-3 were characterized with FE-SEM, N2 adsorption, ICP-AES, 29Si MAS NMR, FT-IR, Raman and XRD analyses. These results show that the delaminated AMH-3 has a more ordered pore structure than the pristine AMH-3 and that it retains framework crystallinity in the AMH-3 layers. Utilizing AMH-3 with activated micropores, we sought to develop ion exchange membranes that have low vanadium crossover for use in vanadium redox flow batteries (VRBs). Nafion-based composite membrane containing delaminated AMH-3 (D-AMH-3) layer was prepared by solution casting and hot pressing. The membrane structure was analyzed by FE-SEM and EDS, revealing a sandwich-type structure that included double Nafion outer layers and a central D-AMH-3 layer. The Nafion/DAMH-3 membrane was employed as an ion exchange membrane for VRB application, and the vanadium permeability and single cell performance were evaluated. The Nafion/D-AMH-3membrane exhibited a lowerVO2+ permeability compared to Nafion, resulting in higher Coulombic efficiency and lower capacity loss per cycle. The results indicated that D-AMH-3 layer is potentially suitable as a permselective barrier for reducing vanadium crossover and improving cell performance. Sulfonated poly(ether ether ketone) (SPEEK) is a potential polymer for replacing Nafion in VRBs. However, at a high degree of sulfonation, SPEEK displays high swelling, poor mechanical stability, and high vanadium crossover. In this study, to improve membrane performance, composite membranes of SPEEK and ultrasonicated D-AMH-3 (U-AMH-3) are prepared with various U-AMH-3 contents and investigated. The physicochemical and mechanical properties, vanadium permeability, and VRB single cell performance of these SPEEK/U-AMH-3 composite membranes are evaluated using various characterization techniques. Interactions between SPEEK and U-AMH-3, and the permselective property of U-AMH-3, result in the composite membranes exhibiting good mechanical properties and low vanadium crossover. Optimal composite membranes gave a VRB that produced a higher charge–discharge capacity, higher cell efficiency, and better capacity retention than that using Nafion. These results indicate that SPEEK-based composite membranes with improved membrane performance, lower vanadium crossover, and good single cell performance were successfully prepared by incorporating U-AMH-3.