Preparation And Separation Performance Of Polydimethylsiloxane-based Hybrid Membranes

Due to the decline of fossil fuel reserves and the negative impact on the environment,people are forced to find new alternative energy.Biofuels are non-toxic,biodegradable and carbon-neutral which is a clean and sustainable alternative to fossil energy.In the production of bioalcohols（butanol and ethanol）via fermentation,the accumulation of fermentation products has an inhibitory effect on microbial cells,resulting in low product yield and productivity,which in turn leads to high energy consumption in the process of product separation and purification.Pervaporation is one of the most promising methods for recovering products from fermentation broth.It has the advantages of simple operation,non-toxic to cells and less energy consumption.Polydimethylsiloxane（PDMS）has good stability and resistance to organic solvents,which is the current hydrophobic reference material for pervaporation.However,the bottleneck of PDMS homogeneous membrane is the low permeation flux and separation factor.In this paper,based on the solution-diffusion model,a series of PDMS-based hybrid membranes were designed and prepared by combining nano materials（zeolite and carbon nanotube（CNT））and using different membrane preparation methods.The separation performance of PDMS based hybrid membranes was successfully improved,and the mechanism of performance improvement was discussed.The main research results are as follows:Firstly,zeolite/PDMS hybrid membranes with different zeolite loading weights were prepared by blending filling method.At 80℃,the separation factor of zeolite/PDMS hybrid membrane was 33.0,and the butanol concentration was 334.6 g/L,which increased with the increase of zeolite loading weights and the temperature of feed solution.When the hybrid membrane was used in the coupling process of pervaporation and fed-batch acetone-butanol-ethanol（ABE）fermentation,the productivity（0.29 g/L h）and yield（0.2 g/g）of butanol were increased by 16.0%and 11.1%,respectively,compared with the batch butanol fermentation,and the separation performance of hybrid membrane was stable.Secondly,by growing CNT array and filling PDMS polymer in situ,the vertically aligned（VA）CNT（open-end）/PDMS hybrid membrane with hamburger-like structure was prepared.The mass fraction of CNT in the hybrid membrane was 37.4 wt%.The elongation at break（89.9%）and tensile strength（2.6 MPa）of the hybrid membrane were significantly improved.The CNT array in the hybrid membrane had good permeability.The hybrid membrane had excellent pervaporation and vapor-stripping-vapor permeation performance for separating ethanol and butanol.The density functional theory（DFT）calculation showed that the adsorption energy of butanol,ethanol,and water molecules on the inner wall of CNT tube was greater than that on the outer wall of CNT tube and PDMS chain,which proved that the permeation molecules were preferentially transported in CNT tube.The hybrid membrane had pervaporation-ultrafiltration diffusion mechanism,which was the key to significantly improve the separation performance.Thirdly,compared with the PDMS homogeneous membrane,the VA CNT（open-end）/PDMS hybrid membrane had higher butanol solubility(18.3（J/cm³）^1/2)and butanol diffusion coefficient（5.62 × 10-11 m2/s）,which improved the total flux（1093.7 g/m2 h）and separation factor（37.6）of butanol separation by pervaporation.The hybrid membrane also showed high selectivity and flux of butanol in aqueous ABE solution,fermentation medium,and fermentation broth.During 150 h continuous separation of fermentation broth,the hybrid membrane showed stable separation performance.In addition,the effects of CNT structure and high temperature annealing treatment on the performance of the hybrid membrane were studied.The results showed that the high temperature annealed VA CNT（open-end）/PDMS hybrid membrane had the best phenol separation performance（total flux 322.9 g/m2 h;separation factor 13.9）,the single gas permeability of CO2（7401 Barrers）and N2（346 Barrers）and the separation performance of binary CO2/N2 mixture.The DFT calculations showed that the adsorption energy of gas/liquid molecules on the inner wall of CNT tube was greater than that on the outer wall of CNT tube and PDMS chain,indicating that gas/liquid molecules preferentially chose the inner channel of CNT tube for adsorption and diffusion.In addition,due to the accordion-like structure of CNT array network,the hybrid membrane had excellent and stable conductivity in the state of stretching or twisting.Finally,a large-area VA CNT（open-end）/PDMS hybrid membrane was fabricated by module splicing method.The total flux and separation factor of the hybrid membrane could reach 671.6 g/m2 h and 9.0 at 80℃.In addition,when hybrid membrane was used in the coupling process of pervaporation and fed-batch ethanol fermentation,the productivity（2.34 g/L h）and yield（0.47 g/g）of ethanol were increased by 9.3%and 4.9%,respectively,compared with the batch ethanol fermentation,and the hybrid membrane showed high efficiency and stable separation performance.In conclusion,the results above of the study show that hybrid membranes have good application prospects in the fields of bioalcohols fermentation,wastewater treatment,gas separation,and flexible conductors.The study will provide a new idea for the nanoscale design of efficient separation membranes and the preparation and application of large-area CNT arrays/PDMS membranes.