ZIF filled PDMS mixed matrix membranes for separation of solvent vapors from nitrogen
Graphical abstract
Introduction
The production in many processes often releases waste air streams containing a large amount of volatile organic compounds (VOCs) [1,2]. It is essential to recover the VOCs from waste air streams not only due to environmental issues but also to recover high value chemicals.
Ethyl acetate, methanol, ethanol, n-propanol and isopropanol are widely used solvents in defense, electronic, textile and rubber industries. One of the industries that uses large amounts of those solvents is packing and printing industry. The solvents, which evaporate from the surface of polymeric packing films during drying process, are swept away to improve the indoor air quality. The VOCs in air are either disposed to atmosphere or separated from air by adsorption-based processes using activated carbon as adsorbent [3].
Following the adsorption period, the adsorption beds are regenerated by increasing temperature, decreasing pressure or flowing sweep gas, which is usually steam or nitrogen. The VOCs swept from the adsorption column are condensed and fed to the distillation columns to separate the VOC mixture into its components. The condensation, which often requires cryogenic temperatures due to low partial pressure of VOCs in N2, is an expensive step in the solvent – recovery process. The efficiency of the process can be enhanced by increasing the partial pressure of VOCs in N2 so that condensation can be carried out at higher temperature. Membrane-based separation process can be effectively used to increase the partial pressure of VOCs.
Rubbery polymers are commonly used to separate VOCs from air/N2 streams owing to their selectivity to VOCs over non-condensable gases such as N2 [[4], [5], [6], [7]]. Polydimethylsiloxane (PDMS) has been mostly used among the rubbery polymers because of its high selectivity and permeability [2,[5], [6], [7], [8]].
Mixed matrix membranes (MMMs) are anticipated to exhibit better separation performance than pure polymeric membranes by combining the desirable properties of polymers and high selectivity/permeability of inorganic filler materials [4,9]. Zeolites, metal organic frameworks (MOFs), zeolitic imidazole frameworks (ZIFs) and carbon nanotubes (CNT) are introduced as porous fillers into polymeric membranes. For example, PDMS/ZSM-5 membranes were applied to separate 40 mol % propylene/N2 mixtures at −20 °C with a feed pressure of 1.02 bar. The membranes showed propylene/N2 permeability ratio of 125 for the feed mixtures containing 40-mol% propylene [10]. Tantekin-Ersolmaz et al. [11]. used HZSM-5, NaZSM-5, 4A and 5A type zeolites in PDMS for separation of n-pentane from i-pentane. The zeolite/polymer interface has a crucial role in the membrane performance so that n-pentane/i-pentane ideal selectivities mostly decreased with increasing zeolite content of membranes. In another study, Kim et al. [12] prepared PDMS based membranes filled with nanosize silica. Incorporation of silica raised the permeance of propylene from 31 GPU to 36 GPU in the separation of 15% vol. propylene/85% vol. N2 mixture while the propylene/N2 selectivity increased from 5.2 to 7.3.
In recent years, ZIFs have often been used in the preparation of MMMs composed of rubbery polymers, especially for the separation of permanent gases like CO2 and CH4. To the best of our knowledge, very limited number of studies have been reported on the separation of solvent vapors by ZIF filled MMMs. Only Fang et al. [4] used ZIF-8/PDMS MMMs for separation of propane from N2. The propane/N2 selectivity of 10 wt % ZIF-8 loaded PDMS MMMs was higher by 38% than that of pure PDMS membrane. Apparently the potential of ZIF filled MMMs should be further investigated in the separation of VOCs from permanent gases.
In this study, five members of ZIF family were used as filler in PDMS membranes for the separation of solvent vapors from nitrogen. ZIF-8, ZIF-L, ZIF-67, ZIF-7, and ZIF-71 were selected as filler because of their morphology, structure and pore size (Table 2). All ZIFs were characterized with XRD, SEM and N2 adsorption. ZIF-X filled PDMS membranes were used for the recovery of alcohol (methanol, ethanol, n-propanol and isopropanol) and ester (ethyl acetate) vapors from N2, which are often used in the packing-and-printing process. This study introduces and assesses the potential of ZIF-X/PDMS MMMs in the recovery of different solvent vapors from N2 for the first time in the literature.
Section snippets
Materials
For the synthesis of ZIFs, 4,5-dichloroimidizole, benzimidazole (Bim), 2- methylimidazole (Hmim), zinc acetate, zinc nitrate hexahydrate (Zn(NO3)2·6H2O), N,N-dimethyl formamide (DMF) and chloroform were purchased from Sigma–Aldrich. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was obtained from MCB.
Solvents that are used in membrane preparation and separation experiments are hexane, methanol (MeOH), ethanol (EtOH), n-propanol (PrOH), isopropanol (IPA) and ethyl acetate (EtAc). All solvents were
Characterization of ZIFs
Fig. 2 shows the XRD patterns of synthesized ZIFs with reference patterns as bar graph. The peak positions and relative intensities match completely with the corresponding reference patterns [[15], [16], [17], [18]], indicating that all ZIFs were successfully synthesized with high crystallinity and purity for use in mixed matrix membranes as microporous filler.
Fig. 3 shows the SEM images of as-synthesized ZIFs. Each type of ZIF has its own characteristic morphology. ZIF-8 has hexagonal
Conclusions
Five types of ZIFs, namely ZIF-7, ZIF-8, ZIF-67, ZIF-71 and ZIF-L, were used as filler to prepare PDMS membranes with 20% ZIF-X loading. The ZIF-X particles were dispersed in the PDMS using hexane as solvent. All ZIFs were uniformly dispersed in the membrane and completely covered with PDMS so that highly selective ZIF-X/PDMS membranes were successfully obtained.
Among the studied ZIFs, membranes prepared with ZIF-L showed the highest VOC permeabilities and VOC/N2 selectivities. The VOC/N2
CRediT authorship contribution statement
Fatma Şahin: Conceptualization, Methodology, Validation, Investigation. Berna Topuz: Methodology, Visualization, Writing - review & editing. Halil Kalıpçılar: Conceptualization, Methodology.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the Scientific and Technological Research Council of Turkey (TUBITAK) and METU Graduate School of Natural and Applied Sciences Research Fund for their financial support with the grant numbers of 116M438 and BAP-03-04-2015-007 respectively.
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