Nanostructured pervaporation membranes for bioethanol dehydration

Current commercial membranes are applied for drying first generation of bioethanol, but the purification and drying of second and third generation of bioethanol are big challenges due to the impurities (fusel alcohols, organic acids, aldehydes, etc.) that are present in the industrial streams. Those impurities harm the membrane materials, either polymeric or ceramic. Even molecular sieves suffer damages due to these harmful components. Therefore, the development of next generation pervaporation membranes is crucial to make pervaporation more attractive and more competitive.
In the first part of the thesis, we investigate the ability of commercial membranes to trigger a specific behavior under different media. Dehydration of binary methyl acetate–water mixtures under neutral, acidic, and basic conditions was carried out by using PERVAP™ composite membranes based on poly(vinyl alcohol) (PVA) and poly(N-vinylpyrrolidone-co-(2-(dimethylamino)ethyl methacrylate)) P(NVP-co-DMAEMA). The effects of an acid (HCl) and a base (NaOH) on the separation performance of the membrane during the pervaporation process were investigated. The pH-responsive nature of membranes has been confirmed by swelling tests and analysis of the chemical structure of polymeric membranes. In addition, a mechanism of ring-opening of NVP units is proposed and correlated to the changes of membrane separation performance. Such membranes are known to be stable in the presence of impurities such as acetaldehyde. However, the membranes typically exhibit poor performance in ethanol/water separation due to low selectivity which is linked to the use of a commercial copolymer designed for other applications, thus discouraging their use for bioethanol dehydration processes. This motivated the need for customizing the copolymer properties to enhance membrane formulations for specific applications like ethanol dehydration.
In the second part of the thesis, we deepen the investigation of the copolymer. Rather than commercial copolymers, tailor-made poly(N-vinylpyrrolidone-co-(2-(dimethylamino)ethyl methacrylate)) P(NVP-co-DMAEMA) and poly(N-vinylpyrrolidone-co-N-vinylimidazole) P(NVP-co-PNVIm) with defined monomer molar ratio are synthesized via free radical polymerization. The random copolymers are fully characterized and then blended with PVA to investigate their chemical and thermal properties as membrane materials. Composite membranes are further prepared from the PVA/copolymer blends on a porous support, which are evaluated in terms of separation performance for the dehydration of ethanol by pervaporation. The membranes prepared from the blends exhibit up to four times higher water permeances than pristine PVA membrane, albeit the selectivity is slightly lower. Nevertheless, the membranes from blends with a ratio of 95:5 (PVA/copolymer) show improved selectivity and higher permeance values compared to the commercial PERVAP™ 4155–80, especially the blends composed by the copolymers of coPDMAEMA60 and coPDMAEMA20. The membrane prepared from the blend containing the homopolymer coPDMAEMA100 exhibits the highest water/ethanol selectivity and shows stable separation performance throughout the whole long-term stability test, while exposed to acetaldehyde. Thus, this study demonstrates that by synthesizing tailored copolymers (rather using the commercial ones) and blending with PVA, the separation performance of membranes can be significantly improved and tuned for specific dehydration processes. These prototypes have proven their efficiency and stability for second and third generation bioethanol dehydration processes. Thus, a considerable step towards the deployment of these membranes has been made.
In the last part, tailor-made poly(vinyl alcohol)-b-poly(styrene) copolymers (PVA-b-PS) for separation membranes are synthesized by the combination of reversible-deactivation radical polymerization techniques. The special features of these di-block copolymers are the high molecular weight (> 70 kDa), the high PVA content (> 80 wt.%), and the good film-forming property. They are soluble only in hot dimethyl sulfoxide, but through the “solvent-switch” technique, they self-assemble in aqueous media to form micelles. When the self-assembled micelles are cast on a porous substrate, thin-film membranes with higher water permeance than that of PVA homopolymer are obtained. Thus, by using these tailor-made PVA-b-PS copolymers, it is demonstrated that chemical cross-linkers and acid catalysts can no longer be needed to produce PVA membranes, since the PS nanodomains within the PVA matrix act as cross-linking points. Lastly, subsequent thermal annealing of the thin film enhances the membrane selectivity due to the improved microphase separation.