Discovering Unique Behaviour of Molecules Diffusing through the Tight Spaces of Amorphous Silica and Zeolite Membranes

Gases and hydrated ions exhibit unique behaviour inside inorganic materials such as amorphous microporous silica and zeolites, where the pore spaces are only slightly larger than they are. Prof Lin was one of the earliest researchers to master successful fabrications of defect-free microporous inorganic membranes for molecular scale separations of gases and liquids. This inspiring early work led to a collaboration starting 12 years ago to explore molecular sieving of gases through tailored pore size amorphous microporous silica, and an even newer topic on the selective behaviour of hydrated ions in zeolite cage structures.

The pores of amorphous microporous silica are unique in their ability to be tailored to selectively permeate small molecules like He and H₂, while reject only slightly larger CO₂, N₂, CH₄ and CO, implying these materials have a pore size of around 0.3nm [1]. This size is challenging for most analytical techniques to measure, but with positron annihilation lifetime spectroscopy (PALS), the hierarchical structure having pore sizes peaking at 0.3nm, 0.8nm and 1.2nm was revealed for the first time, together with confirmation of the proposed silica network assembly mechanisms upon thermal treatment [2].

More recently, a study was extended to hydrated ions in zeolite structures. The first known studies on this commenced with MFI-type zeolite membranes made by in-situ crystallization, demonstrating a unique aqueous ion sieving effect, for example Na⁺ rejection of 77% whereas Mg²⁺ rejection was reported at 88% [3]. Despite being significantly more expensive than traditional well established desalination technologies, MFI-type zeolites interact uniquely and specifically to hydrated ions yielding potentially more useful effects. For example, monovalent ions such as Na⁺ and K⁺ more easily enter the zeolite intrinsic pores, altering the unit cell dimensions, while divalent ions such as Ca²⁺ and Mg²⁺ mostly interacted within the intercrystalline pores [4]. Further tailoring of the ion behavior was achieved through Si/Al ratio of the zeolite [5], while increasing temperature from 20°C to 90°C saw a unique ability to reversibly trigger selective passage of K⁺ [6]. Meanwhile, larger multivalent ions Al³⁺ and Fe³⁺ showed strong binding uniquely into the intercrystalline pores, offering the ability to block grain boundary defects and increase rejection of smaller ions such as Na⁺ [7]. Although zeolite membranes perform well as extremely robust desalination membranes [8], their higher cost implies viable outcomes in niche areas such as sensors or targeted high value ion separations.

References

1. Diniz da Costa, J.C., Lu, G.Q., Rudolph, V., and Lin, Y.S., Novel molecular sieve silica (MSS) membranes: characterisation and permeation of single-step and two-step sol-gel membranes. Journal of Membrane Science, 2002. 198(1): p. 9-21.
2. Duke, M.C., Pas, S.J., Hill, A.J., Lin, Y.S., and Diniz da Costa, J.C., Exposing the Molecular Sieving Architecture of Amorphous Silica Using Positron Annihilation Spectroscopy. Advanced Functional Materials, 2008. 18(23): p. 3818-3826.
3. Li, L., Dong, J., Nenoff, T.M., and Lee, R., Desalination by reverse osmosis using MFI zeolite membranes. Journal of Membrane Science, 2004. 243(1): p. 401-404.
4. Zhu, B., Zou, L., Doherty, C.M., Hill, A.J., Lin, Y.S., Hue, X., Wang, H., and Duke, M., Investigation of the effects of ion and water interaction on structure and chemistry of silicalite MFI type zeolite for its potential use as a seawater desalination membrane. Journal of Materials Chemistry, 2010. 20: p. 4675-4683.
5. Zhu, B., Doherty, C.M., Hu, X., Hill, A.J., Zou, L., Lin, Y.S., and Duke, M., Designing hierarchical porous features of ZSM-5 zeolites via Si/Al ratio and their dynamic behavior in seawater ion complexes. Microporous and Mesoporous Materials, 2013. 173(0): p. 78-85.
6. Zhu, B., Hong, Z., Milne, N., Doherty, C.M., Zou, L., Lin, Y.S., Hill, A.J., Gu, X., and Duke, M., Desalination of seawater ion complexes by MFI-type zeolite membranes: Temperature and long term stability. Journal of Membrane Science, 2014. 453(0): p. 126-135.
7. Zhu, B., Hu, X., Shin, J.-W., Moon, I.-S., Muraki, Y., Morris, G., Gray, S., and Duke, M., A method for defect repair of MFI-type zeolite membranes by multivalent ion infiltration. Microporous and Mesoporous Materials, 2017. 237: p. 140-150.
8. Zhu, B., Myat, D.T., Shin, J.-W., Na, Y.-H., Moon, I.-S., Connor, G., Maeda, S., Morris, G., Gray, S., and Duke, M., Application of robust MFI-type zeolite membrane for desalination of saline wastewater. Journal of Membrane Science, 2014. 475: p. 167-174.