2D Siloxene Hybrid Membranes for Liquid Separations

: (Alternative Format Thesis)

Abstract

This work investigates the two-dimensional (2D) siloxene material for application in the field of membrane technology, with the main aim of siloxene synthesis, exfoliation and to fabricate nanostructured mixed matrix membranes (MMMs) and hybrid membranes for liquid separation purposes.

Water supply is threatened across the globe due to water scarcity and water pollution. Existing membranes, polymeric and ceramic in nature have reached stasis in terms of their selectivity, permeability, and low resistance to fouling. Researchers have made advancement to prepare membranes by incorporating 2D nanomaterials such as grapheme oxide (GO) and reduced grapheme oxide (rGO) in polymeric materials. These membranes have made considerable contributions in terms of separation performance enhancement in pressure driven membrane processes such as nanofiltration (NF). However, due to the agglomerating, hydrophobic nature of these nanomaterials, and polymeric material, resulting membranes are fouled easily. This therefore results in the need to find more hydrophilic two-dimensional (2D) nanomaterials with high surface charge to develop novel membranes.

The successful synthesis of 2D siloxene was confirmed through transmission electron microscope (TEM) analysis, energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR). Bulk siloxene was exfoliated via liquid phase exfoliation (LPE) method in six different solvents for the first time to achieve scalable and stable siloxene exfoliation dispersions. The results revealed high degree of exfoliation in water and green solvent Cyrene but restacking of 2D siloxene nanosheets was observed in case of water whereas dispersions were more stable in Cyrene over a longer period. LPE in Cyrene at 300 W for 6 hours resulted in average lateral sizes ~ 0.25 μm with thickness Following the aforementioned exfoliation study of siloxene, the incorporation of 2D siloxene to fabricate mixed matrix membranes (MMMs) was investigated. The exfoliated siloxene nanosheets were dispersed in 17.5 wt. % polyvinylidene fluoride (PVDF) to prepare MMMs with increased performance and rejection properties. Different loadings of the siloxene nanosheets (0.0 wt. %, 0.025 wt. %, 0.050 wt. %, 0.10 wt. %, 0.15 wt. %) were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare casting dope solutions followed by phase inversion immersion technique. After fabrication, the membranes were characterized for various properties such as surface morphology, hydrophilicity and cross flow pure water permeance. The filtration results revealed that among all the compositions tested, the best performing membrane PVSi-075 showed NF performance with rose Bengal rejection above 90% and pure water permeance of 22 ± 2 L m-1 h-1 bar-1. The molecular weight cut-off (MWCO) of the PVSi-075 membrane was determined to be around 530 Da and it showed stable hexane permeance of 11 L m-1 h-1 bar-1 for 24 h. These performances were attributed to the compaction of polymer chains with low fractional free volume and high ordering caused by the incorporation of 2D siloxene nanosheets, which shows the promise of their use in nanofiltration processes under harsh conditions.

Siloxene nanosheets were further used to prepare 2D lamellar membrane over a porous support. A novel hybrid siloxene membrane was obtained via vacuum filtration method by introducing cross-linkages in between 2D siloxene nanosheets and porous polymeric support. The NySi-10 membrane exhibited excellent water permeance of 58 ± 4 L m-1 h-1 bar-1. The membrane showed high rejection (>94%) for dyes with molecular weight above 690 Da. These results show their potential in filtration processes.

Date of Award	29 Mar 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Davide Mattia (Supervisor) & John Chew (Supervisor)