SIFSIX-3-Zn/PIM-1 mixed matrix membranes with enhanced permeability for propylene/propane separation
Graphical abstract
Introduction
In the petrochemical industry, propylene is one of the most principal raw materials. In 2016, the global propylene production was 99 million tons, and by 2025, the demand growth rate of propylene is expected to 4.0% per year [1]. Normally, high purity (≥99.5%) propylene is produced via an extremely energy-intensive process of low temperature distillation, which involves more than 200 stages due to the contiguous boiling points of propylene and propane [2]. Membrane-separation processes have been considered as an attractive alternative due to their low energy consumption, ease of operation, small footprint and easy scale production. So far, regarding the propylene/propane separation, there have been many different types of membranes researched, including polymeric membranes [3,4], MOF membranes [5,6], facilitated transport membranes [7,8] and carbon molecular sieve(CMS) membranes [9,10]. However, all of these membranes suffer from several limitations more or less. For example, most of polymeric membranes are processable, robust and inexpensive, but these generally suffer from the well-known upper bound trade-off curve for C3H6/C3H8 separation meanwhile exhibiting low reliability and durability [3]. MOF membranes exhibit high selectivity and permeability, but are difficult to fabricate on a pilot-plant scale due to uncontrollable inter-crystalline defects [11]. Facilitated transport membranes exhibit sufficiently high separation selectivity, but can be poisoned by trace amount of impurities in the feed stream [1,12]. CMS membranes can simultaneously achieve the high permeability and high selectivity performance, however, they currently are too brittle to scale up commercial production.
One attractive approach to overcome these obstacles is to incorporate dispersed inorganic fillers within bulk continuous polymeric matrix to fabricate mixed matrix membranes (MMMs), which combine the attractive properties of polymer and inorganic microporous materials. Recently, metal-organic frameworks (MOFs)-based MMMs are more preferable to traditional MMMs including zeolites [13], silica [14,15] and carbon materials [16,17], due to high porosity and surface area [18], fine-tuning aperture sizes, adjustable pore surface properties [19,20] and reasonable chemical stabilities [21]. MOFs' high pore volumes and tunable aperture’ sizes can endow the resulting MMMs an enhanced permeability and selectivity [[22], [23], [24]]. Moreover, as compared to conventional fillers which require additional compatibilization modification to adjust filler/polymer interface [25,26], MOFs can be blended directly into polymer matrix to produce defect-free MMMs due to excellent affinity between organic moiety in MOFs and organic polymers [27]. In order to improve C3H6/C3H8 separation performance of polymer membranes, MOFs with tunable diffusibility or solubility of gases in the membrane have been selected as filler materials. In recent years, Yu et al. [28] studied the ZIF-8/PVAc MMMs for C3H6/C3H8 separation. They observed that the separation performance was remarkably enhanced with an increase in ZIF-8 nanoparticles loading up to 39 wt%. Japip et al. [29] demonstrated that 6FDA-Durene-based MMMs embedded with ZIF-71 nanoparticles exhibited significant enhancement of C3H6 permeability without remarkable compromising the ideal C3H6/C3H8 selectivity. Recently, Liu et al. [30] observed that the NbOFFIVE-1-Ni/6FDA-DAM MMMs displayed 9 times higher C3H6 permeability than that in pure 6FDA-DAM membrane due to NbOFFIVE-1-Ni nanoparticles facilitating C3H6 transport. However, there are few mixed matrix membranes with permeability exceeding 1000 Barrer, which hinders their industrial application. Therefore, the development of high permeability mixed matrix membranes has become one of the research hotspots in the field of gas separation membranes.
Recently, the SIFSIX-3-Zn has received attention due to its excellent performance for acetylene capture from ethylene [31], biogas upgrading [32] and post-combustion carbon dioxide capture [33]. In the SIFSIX-3-Zn material, two-dimensional nets of pyrazine and zinc ion node are pillared with SiF62− anions in the third dimension to construct three-dimensional coordination networks that present primitive cubic topology [31,34]. SIFSIX-3-Zn material is capable of molecularly discriminating C3H6 molecular (∼4 Å) from C3H8 molecular (∼4.3 Å) [5,29] due to its effective pore aperture size (∼4.2 Å) [31]. In addition, SIFSIX-3-Zn material may exhibit novel C3H6 capture performance because the geometric disposition of SiF62− moieties may enable preferential binding of propylene molecules. Given the differences in aperture size and gas affinity between C3H6 molecular and C3H8 molecular, SIFSIX-3-Zn material incorporated into a polymer matrix as the inorganic filler may enhance the gas separation performance of the resulting membrane as it can provide external delivery channels for C3H6 molecular.
In this work, a rigid linear ladder polymer PIM-1 was selected, which possesses a high free volume as well as excellent phy-chemical properties [35]. SIFSIX-3-Zn nanoparticles were incorporated into polymeric matrix via a facile mixed-matrix approach. SIFSIX-3-Zn nanoparticles with high porosity and surface area can significantly increase the fractional free volume of resulting MMMs which determines the level of gas permeability. Furthermore, SIFSIX-3-Zn nanoparticles with size-sieving and C3H6-philic dual interactions may provide channels which greatly facilitate propylene molecular transport in the SIFSIX-3-Zn/PIM-1MMMs. Hence, SIFSIX-3-Zn/PIM-1MMMs could be expected to break through the bottleneck of low permeability in the industrial application field of C3H6 purification. The free volume fraction, structural morphology and separation performance of SIFSIX-3-Zn/PIM-1MMMs were characterized and tested with Positron annihilation lifetime spectroscopy (PALS), scanning electron microscopy (SEM) and pure C3H6/C3H8 gas, respectively.
Section snippets
Materials
N, N-dimethyl formamide (99%), chloroform (99.5%), methanol (99.5%), ethanol (99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd, China. CDCl3 (99.8%), anhydrous potassium carbonate (99.5%) was purchased from Alfa Aesar, UK. Tetrafluoroterephthalonitrile (TFTPN), zinc hexafluorosilicate hydate (ZnSiF6.xH2O, 99%), 4,4′-bipyridine (C10H8N2, 98%), pyrazine (C4H4N2, 99%) were purchased Sigma Aldrich, Germany and used without further purification. 5, 5′, 6, 6′-tetrahydroxy-3, 3, 3′,
Characterization of SIFSIX-3-Zn nanoparticles
The SIFSIX-3-Zn nanoparticles with sizes apparently far smaller than 1 μm were synthesized by the previous reaction at room temperature for 3 days. As shown in Fig. 1a, the SIFSIX-3-Zn nanoparticles with small sizes are more suitable for the preparation of high-quality mixed-matrix membranes with non-defective morphology due to good filler/polymer compatibility and uniform distribution of filler particles in polymer matrix [27,39]. The structure of the synthesized submicron particles was
Conclusions
We have demonstrated a simple strategy to significantly enhance C3H6 permeability of PIM-1 without compromising its selectivity and simultaneously improve long-time stable separation performance by the incorporation of fluorinated MOF nanoparticles. The sample embedded with 10 wt% SIFSIX-3-Zn nanoparticles shows an optimal enhancement of C3H6 permeability from 1701.9 Barrer to 4012.1 Barrer with simultaneously the ideal C3H6/C3H8 selectivity increasing from 3.6 to 7.9, which is placed in
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. U1704139 and 21376225), Training Plan for Young Backbone Teachers in Universities of Henan Province (2017GGJS002) and China Postdoctoral Science Foundation (2018M642796). The authors gratefully thank Center of Advanced Analysis & Computational Science, Zhengzhou University for help with the characterization.
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These authors contributed equally to this work.