Two-Dimensional Zirconium-Based Metal-Organic Nanosheets for Separation Applications

Abstract

The ability to separate mixtures of chemical species is key to technologies as diverse as drug delivery, water purification and carbon capture. Two-dimensional zirconium-based metal-organic nanosheets (MONs) have well-defined pore size, tunable functionalities, good water, thermal and chemical stability, making them ideal materials for use in separation applications. However, the separation capability of MONs when processed into membranes is not well-studied. In this thesis, Zr-BTB (BTB = 1,3,5-benzenetribenzoate) MONs were synthesised and processed into gels and membranes for use in a variety of solution and gas phase separations. In Chapter 2, Zr-BTB MON suspensions were shown to form self-healing gels with a house of cards structure through a facile centrifugation method. Molecular cargoes could be loaded into the gels through centrifugation of mixtures composed the Zr-BTB MON suspensions and the cargo solutions. A significantly higher loading was observed for the charged cargoes such as methylene blue (MnB), methyl orange (MO), and brilliant blue G (BBG), compared to the neutral ones such as trans-anethole (T-Ane) and carbamazepine (CBZ). Neutral T-Ane cargoes smaller than the pore size of Zr-BTB were found to diffuse more rapidly out of the gels than the larger CBZ cargoes whilst charged cargoes (MnB, MO and BBG) were found to release much more slowly than the neutral ones. This was attributed to the different diffusion pathways open to different species with the porous nanosheets acting like “fishing nets” which smaller species could pass through whilst larger species had to travel around and charged species stuck to them. In Chapter 3, Zr-BTB MON suspensions were spin-coated onto polyethersulfone (PES) supports to fabricate composite membranes. The fabricated membranes showed a high permeability toward small negatively charged dyes such as MO and a high rejection rate of up to ~90% toward larger dyes such as BBG. Deposition of the MONs onto the PES supports also increased the adsorption of positively charged dyes such as MnB on the membranes and finally improved their rejection rates toward MnB. In Chapter 4, Zr-BTB MONs were incorporated into Matrimid and PEBAX polymers to fabricate mixed matrix membranes (MMMs) for use in the separation of CO2 and N2. It was found that the incorporation of 5 wt% of MONs into the polymers decreased the CO2/N2 separation selectivity of the membranes. The MON suspensions were also processed into composite membranes by depositing them onto polyacrylonitrile supports. However, due to the relatively large pore size of the MONs (5.4 Å) compared to the kinetic diameters of the small gas molecules (CO2: 3.3 Å, N2: 3.68 Å) as well as the defects in the fabricated membranes, they showed high gas permeability, but almost no CO2/N2 separation selectivity. In both Chapters 3 and 4, MONs composed of Zr6 clusters and 2′-amino-5’-(4-carboxyphenyl)-[1,1’:3′,1′′- terphenyl]-4,4′′-dicarboxylate (BTB-NH2) or 4,4’,4’’-s-triazine-2,4,6-triyl-tribenzoate (TATB) ligands were also used to fabricate membranes. However, in comparison with Zr-BTB, the use of Zr-BTB-NH2 and Zr-TATB did not significantly improve the separation performance of the membranes toward organic dyes and gases, which could be attributed to the isoreticular structure of the three MONs, giving them the same pore size. Overall, this thesis demonstrates that two-dimensional zirconium-based MONs can be used to prepare gels for use in the selective loading and differential release of small molecules, and the MONs can also be processed into different types of membranes for use in the separation of dyes and gases. Through functionalisation of the MONs to tune their pore size and surface properties, we anticipate progress in the separation of small dyes, gas molecules and salts