The exploration of bio-renewable solvents in membrane fabrication for applications in alcohol recovery and water purification

Membrane separation is one of the most widespread sustainable and ecological technologies for purifying and separating waste streams. This process can substitute thermal separation processes such as distillation, evaporation, and crystallisation. Membrane separation has been proven to be promising for liquid separations due to the high efficiency, low operation cost and energy-saving performance in numerous applications of this process. Nanofiltration (NF) is a pressure-driven membrane separation process that employs membranes for molecular separation and purification in liquid applications. Organic solvent nanofiltration (OSN) is a membrane process for molecular separation in harsh organic media. It excludes molecules dissolved in organic solvents based on shape, charge and size, allowing the reuse of solvents. NF membranes are typically produced as thin-film composite (TFC) membranes comprising of a thin porous selective layer typically derived from amines and acyl chlorides deposited on a porous substrate created from petroleumderived synthetic polymers. The materials used in both layers are toxic, hazardous, petroleum-derived and non-biodegradable contributing to landfill and pollution. In recent years, works reported in the literature have been invested in embedding sustainability into membrane fabrication through the use of bio-renewable solvents and the use of sustainable raw materials. Although considerable progress has been made in the past few years with the production of green solvents from renewable feedstock, the use of these solvents for membrane fabrication has not been fully explored as new solvents are constantly emerging. The work in this thesis aims to replace conventional materials used for membrane fabrication to create membranes that have lower environmental impact and health and safety concerns through the use of bio-renewable solvents and sustainable fabrication methods. The work in this thesis extends current research in green solvents through the exploration of Cyrene™ and 2-Methyltetrahydrofuran (2-MeTHF) for TFC membrane fabrication, two solvents that have not yet been explored. The work also investigated the application of these bio-renewable membranes in aqueous and organic solvent nanofiltration. The thesis starts with the development of support that is stable in a wide range of organic solvents, allowing it to become the foundation of a thin film composite membrane that could be utilised in OSN applications. A comparative study was carried out, and supports were produced using Dimethylformamide, (DMF) (the conventionally used solvent) which were compared to supports produced using Cyrene™ (the bio-renewable alternative). The study looked at investigating the effects of using a bio-renewable solvent on the resultant support characteristics. The thermal and chemical stability, solvent filtration and morphology of the two supports were investigated. An important aspect considered in this project was the operating conditions for membrane fabrication. Traditional membranes are produced using complicated lengthy sixteen-hour energy-intensive procedures, the work in this project aimed to produce membranes using benign room temperature conditions. The fabrication of polyimide (PI) the most conventional polymer used in OSN membrane fabrication and Cyrene™ were explored and the combination of the polymer/solvent/nonsolvent was unsuccessful in creating support. The polymer was changed to a renewable polymer, cellulose acetate (CA), and quick room temperature fabrication methods for 90 minutes were utilised to produce supports stable in harsh organic environments. The bio-renewable supports exhibited excellent permeance of solvents, and a 115% increase in water permeation was experienced compared to the traditional support produced using DMF. Protocols in this work were established that produced supports that maintained structural integrity and excellent stability in DMF immersion at 100 °C and performed well in different solvent environments. The second part of this thesis was to fabricate polyamide, the selective layer used in a TFC membrane using 2-MeTHF. The polymer was studied and fully characterised and compared to the traditional polyamide produced using the petroleum-derived n-hexane solvent. The two polyamide layers were deposited onto the support produced using Cyrene™, and a TFC membrane was created using only bio-renewable solvents. Ethanol permeance of 2.5 L m⁻² h⁻¹ bar⁻¹ was experienced when using the n-hexane derived polyamide, and this increased to 11.2 L m⁻² h⁻¹ bar⁻¹ when 2-MeTHF was utilised as the solvent for interfacial polymerisation. A further 900% increase of ethanol permeance was experienced after DMF activation, reaching 25 L m⁻² h⁻¹ bar⁻¹. A detailed study was carried out to test for OSN applications with a range of dyes with varying molecular weights and charges and the molecular dye rejection rates of the bio-renewable TFC membrane reached 98%, higher than the n-hexane derived polyamide TFC membrane at 94%. The membrane produced solely from bio-renewable solvents outperformed current state-of-the-art membranes and mixed matrix membranes that incorporate fillers into the membrane for enhanced separation performance. The final part of the thesis explored the potential of the bio-renewable TFC membranes being utilised in aqueous NF applications using a feed solution of different salts. The performance capabilities of the bio-renewable membrane were compared with a commercially available NF TFC membrane, NF 270. The bio-renewable TFC membrane experienced a higher water permanence compared to the NF 270 TFC membrane at 17.8 L m⁻² h⁻¹ bar⁻¹ and 11.8 L m⁻² h⁻¹ bar⁻¹ respectively. Further to this, magnesium chloride rejection for the bio-renewable TFC membrane reached 39% while the NF 270 TFC membrane reached 36%. The bio-renewable membrane outperformed the commercial membrane, and this opens up an interesting avenue of research, as the use of sustainable materials and benign operating conditions could compete with the current state-of-the-art membranes. The results presented in this thesis make a valuable contribution to the exploration of the two bio-renewable solvents for membrane fabrication. The fabrication strategies developed in this work are time-saving, energy-efficient and cost-effective and protocols were established that have improved health, safety and environmental impact. Nextgeneration membranes can be fabricated with sustainable materials to produce membranes that potentially have higher through-puts and require less energy for separations to occur which could potentially transform polymer membrane fabrication into a more sustainable process.