Mixed matrix membranes with HF acid etched ZSM-5 for ethanol/water separation: Preparation and pervaporation performance
Graphical abstract
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Highlights
► MMMs were prepared by incorporation of surface etched ZSM-5 with HF acid into PDMS membranes. ► Effect of the zeolite surface morphology on interfacial integration and pervaporation performance of ZSM-5/PDMS MMMs. ► The pervaporation mechanism of MMMs was investigated based on sorption experiment. ► MMMs filled with etched ZSM-5 showed much better separation performance than that filled with non-etched ones.
Introduction
Due to an emerging scarcity of the fossil fuels and an associated increase of the oil prices, the production of ethanol biofuels from renewable biomass resources has been received increasing attention [1], [2]. A traditional fermentation process generally produces dilute ethanol solution with concentration less than 8–10 wt%, because the higher ethanol concentration will inhibit the growth of yeast cells and even kill them which will stop the fermentation process. To improve ethanol productivity, distillation was usually used to remove ethanol from fermentation continuously, which led to high cost directly in the production of biomass ethanol [3], [4], [5], [6], [7]. Compared with distillation, pervaporation is considered to be an energy-efficient alternative for recovering ethanol from their dilute solutions in fermentation broths [5], [6], [7], [8], [9].
For the selective removal of ethanol by pervaporation, membrane has the decisive influence on separation performance. Many research showed that widespread commercial acceptance of polymer membrane for ethanol recovery has been hindered by a number of performance-limiting phenomena, such as poor pervaporation stability, low selectivity and/or low permeability, as well as permeability/selectivity tradeoff property [10], [11], [12], [13], [14], [15], [16]. However, mixed matrix membranes (MMMs) have been proved to be a promising solution to the inherent permeability/selectivity tradeoff of pure polymer membranes [17], [18], [19]. MMMs were usually defined as the incorporation of a solid phase into a continuous polymer matrix [20]. The objective was to use the solid phase in a composite membrane to provide a means to overcome the tradeoff performance based on the higher selectivity and/or permeability of solid phase than the polymer matrix. Porous zeolite and hydrophobic PDMS were usually used for the preparation of MMMs and applied in the ethanol/water separation by pervaporation. And several progresses have been made in the developing MMMs for ethanol recovery. However, the maximum separation performance of MMMs was restrained by a variety of interfacial defects and nonidealities, including leaky interface, zeolite aggregation, defect and barriers in zeolite [17], [18], [19], [20].
To overcome the interfacial defects and nonidealities, most research focused on the modification of surface physiochemical properties of raw zeolite. Chen et al. [21] studied the effect of HCl acid and steam treatment of silicalite-1 on the pervaporation performance of zeolite filled PDMS membranes. Moermans et al. [22] treated nano-sized zeolites with silane coupling reagent to increase the zeolite loading and dispersion in silicone membranes. We also reported that modification of silicalite-1 with chlorosilanes was very effective to improve integration with PDMS matrix and the selectivity of MMMs for ethanol [23]. All of the modification methods mentioned above were moderate to improve the interfacial integration and pervaporation performance by increasing the organic constitutes on zeolite surface or eliminating impurities in zeolite inner pores. Few researches focused on the effect of zeolite surface morphology on the interfacial integration and pervaporation performance of MMMs.
In the present work, we first reported the incorporation of surface etched MFI zeolite with HF acid into PDMS membranes and the effect of the zeolite surface morphology on interfacial integration and pervaporation performance of MMMs. Moreover, the sorption and diffusion selectivity of MMMs were also studied to clarify the pervaporation mechanism of MMMs. Besides, the effect of preparation and operation conditions on pervaporation performance of the mixed matrix membranes for separation of ethanol/water mixtures was also investigated in detail.
Section snippets
Materials
α,ω-Polydimethylsiloxanediol (PDMS) was purchased from Beijing Chemical Reagents Corporation, with kinetic viscosity of 50,000 mPa s. Micro-sized ZSM-5 zeolite with a Si/Al ratio of 300 was bought from Nankai University Catalyst Corporation without any other treatment. HF acid (30 wt%), acetone, poly(phenyltrimethoxylsiloxane) (PTMOS), n-hexane and di-n-butyltin dilaurate (DBTOL) were obtained from Beijing Jingyi Chemical Reagents Corporation, and were used as received. The preparation of PVDF
Characterization
Comparison of zeolite surface properties before and after surface etching with HF acid was made based on FT-IR spectra as shown in Fig. 3. In the spectrum of 0F, a broad peak near 3450 cm−1 was observed which was attributed to the terminal hydroxyl groups and hydrogen-bonding adjacent hydroxyl groups. The absorption intensity of hydroxyl groups in 1-5F decreased significantly. In the spectra of 0-2F, organic CH peaks in the region of 2850–3000 cm−1 were observed which attributed to the trace of
Conclusion
A study of ZSM-5 filled PDMS MMMs was extended to probe the effects of HF etching of ZSM-5 on the pervaporation performance of ethanol/water mixtures. It was first shown that the ZSM-5 hydrophobicity and surface roughness was successfully increased. The PDMS MMMs filled with etched ZSM-5 exhibited high tensile strength and swelling resistance due to the tight integration of PDMS and zeolite phases, which was stemmed largely from the intrusion of PDMS polymer into micro-sized pores out of the
Acknowledgements
The authors greatly appreciate the BTBU Young Teachers Scientific Research Foundation (QNJJ2011-25), College Students Scientific Research and Undertaking Starting Action Project (PXM2012_014213_000067), Postdoctor Science Foundation of China (20100470015, 20100480234), and National Natural Science Foundation of China (20906056, 21176135).
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