Novel PTMSP-Silica Pervaporation Membranes for Bio-Alcohol Recovery

The increasing interest in the production of bio-alcohols as alternative for or additive to fossil fuels and as platform molecules for the synthesis of bio-chemicals has led to a revival of fermentation processes. Butanol, for instance, is an important commodity chemical used as a solvent and as a precursor for chemical synthesis. In addition, it holds tremendous promise as a second-generation biofuel as it combines a considerably higher combustion value than ethanol with chemical properties that allow straightforward blending. While most of the current industrial bio-alcohol plants rely on batch fermentation processes and energy-intensive distillation processes to recover the produced alcohols, membrane processes working at ambient conditions offer the possibility to remove the alcohols from the broth during the bioconversion itself. As a result, the fermentation is converted into a continuous production process with in situ recovery of bio-alcohols, thus avoiding any yield reduction caused by product inhibition which normally occurs in batch fermentation processes. Pervaporation can be considered as one of the most promising technologies for process intensification in fermentation processes.

However, in situ product recovery (ISPR) requires robust hydrophobic membranes, combining a high permeate flux with an outstanding selectivity. Most of the commercial hydrophobic pervaporation membranes that have been reported in relation to bio-alcohol capturing are based on PDMS membranes, although their alcohol/water separation factors and permeate fluxes are quite low. Therefore, VITO has been developing an alternative organophylic membrane based on poly[1-(trimethylsilyl)-1-propyne] (PTMSP), a substituted polyacetylene with a rigid backbone combined with bulky trimethylsilyl side groups, resulting in a high free volume fraction. Thanks to its nanoporosity, glassy nature, solubility controlled separation mechanism and pronounced hydrophobicity, PTMSP has great potential in the development of gas and vapour separation membranes. Since Masuda synthesized the polymer for the first time in the 1980s, it remains one of the most permeable polymers. By incorporating hydrophobic silica particles, PTMSP’s free volume can be even further enhanced [1-2].  

In the present work, thin PTMSP-silica nanohybrid films were applied on top of a porous PVDF support, allowing to reduce the thickness of the separating layer down to approximately 2 µm. The PTMSP-silica membranes were tested in the separation of 5 wt% alcohol/water mixtures and benchmarked against two commercial PDMS based membranes [3-4]. With fluxes as high as 10 kg.m^-2.h^-1 and separation factors up to 25, the thin film PTMSP membranes performed much better than commercial PDMS membranes, and experimental membranes with comparable performance have not been reported in literature either. Tests on model butanol/water mixtures further indicated the potential of the developed membranes in alcohol recovery.

Besides testing on model alcohol/water mixtures, long term trials with PTMSP-silica membranes have been conducted using an in-house developed fermentation-pervaporation set-up where the membranes were directly coupled to a continuous two-stage ABE fermentation reactor, using a Clostridium acetobutylicum strain [5]. While the butanol/water separation factor was significantly larger than for PDMS membranes, the tests revealed a steady flux loss, attributed to fouling caused by fermentation by-products. To get insight into the mechanism of this performance decline and to evaluate the effect of these byproducts, pervaporation tests on cell-free ABE model mixtures with progressively increasing complexity have been set up. Parallely, the ageing of the PTMSP membranes under different conditions, as well as its reversibility have been demonstrated through angle-resolved X-Ray photoelectron spectroscopy (AR-XPS) and infra-red spectroscopy (IR). Finally, the integration of an upstream nanofiltration step has been investigated in order to remove the fouling by-products, enabling the pervaporation properties to remain unaffected.

Acknowledgements

The Flemish Government is acknowledged for funding this work in the framework of the Membrane based Product Recovery (MemProRec) project.

References

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