Fluorine-containing polyimide/polysilsesquioxane carbon molecular sieve membranes and techno-economic evaluation thereof for C3H6/C3H8 separation
Graphical abstract
Introduction
A promising membrane in challenging gas separations can be realized through development of carbon molecular sieve (CMS) membranes because of their peculiar bimodal pore structures [1,2]. These peculiar pore structures of CMS membranes can be tuned by the effect of pyrolysis protocol, pyrolysis atmosphere and pre/post treatments [3,4]. The pyrolysis protocol is determined by parameters including the final soaking temperature, soaking times, and ramp rates. The higher soaking temperatures and longer soaking times during pyrolysis are known to cause a decrease in the size of micropore and ultramicropores, resulting in an increase in selectivity and a decrease in permeability [5]. The pyrolysis atmosphere can be controlled by using various types of purge gases. Most notably, engineering the size of ultramicropores via oxygen doping during pyrolysis is an effective method to enhance selectivity [6,7]. Pre/post treatments refer to additional steps that are taken following a precursor membrane fabrication [8]. Richter et al. demonstrated that by performing post-pyrolysis oxygen treatments on CMS membranes, the transport mechanism is changed from a molecular sieving process to a selective surface flow [9].
Meanwhile, engineering novel polymer precursor is also critical to achieve desirable pore structures in CMS membranes for high separation performance. Chu et al. showed that polyimides containing iron complexes show enhanced olefin affinity, resulting in higher olefin/paraffin selectivity [10]. Furthermore, CMS membranes derived from an intrinsically microporous polyimide showed a superior ethylene/ethane selectivity as well as a high ethylene permeability because the collapse of the microporous structure was hindered [11]. Additionally, CMS derived from polymers of intrinsic microporosity (PIM) with beta-cyclodextrin showed a slight increase in propylene permeability and propylene/propane selectivity compared to those derived from the untreated PIM membrane [12]. Other groups have fabricated carbon-zeolite composite membranes in order to combine the advantages of high separation capability as well as chemical/thermal stability [13,14]. These approaches, however, lacked the fabrication of zeolites with pore sizes perfectly fit for separation of gases with miniscule size difference. Park and Lee introduced a series of carbon-silica (C–Si) composite membranes, where the continuous carbon matrix behaves as a molecular sieve while the dispersed SiO2 domains enhanced productivity [15]. Unfortunately, their approach was inappropriate for high C3H6/C3H8 selectivity, albeit substantially improving C3H6 permeability.
Recently, our group demonstrated the fabrication of CMS hollow fiber membranes with a thin selective layer, which was derived from a hybrid polymeric precursor containing a polyimide and a ladder-structured polysilsesquioxane [16]. The rigid double-stranded siloxane backbone suppresses thermal relaxation during pyrolysis, allowing the preparation of highly productive CMS hollow fiber membranes. This has prompted us to explore the effect of LPSQ addition on the olefin/paraffin separation performances of the hybrid CMS membranes.
Here, we investigated the effect of thermo-oxidative crosslinking of siloxanes on the C3H6/C3H8 separation efficiency of CMS membranes derived from a fluorinated PI. The Koros group reported the V-treatment process, which involves the coating of vinyltrimethoxysilane to be used for the suppression of substructure collapse in CMS membranes [17]. While thermo-oxidative crosslinking of siloxanes in CMS membranes is also seen in V-treatment, the excess silica from V-treatment simply acted as an additional resistive layer without enhancing the gas selectivity [17,18]. However, the siloxane components in our CMS membranes may behave as an impermeable dispersed phase in the amorphous CMS matrix, possibly generating a tortuous path for larger penetrants. CO2 physisorption and TEM analysis was employed to characterize the CMS membranes derived from polyimide/polysilsesquioxane dense films. Furthermore, the C3H6/C3H8 single gas separation performance and equimolar mixed gas separation performance of CMS membranes as a function of polysilsesquioxane content was evaluated. In addition, the C3H6/C3H8 single gas separation performance of hybrid CMS hollow fiber membranes were studied in order to evaluate the economic feasibility of membrane processes involving such materials.
Section snippets
Materials
6FDA-DAM:DABA (3:2) (PI) polymers and ladder structured-poly (phenyl-co-pyridylethyl)silsesquioxane with phenyl: pyridylethyl ratio of 6:4 (LPPyr64), LPSQ was synthesized in-house as described in our previous work [16]. Single gases of CO2, N2, CH4, C3H6, C3H8 (purity of 99.999%) and equimolar C3H6/C3H8 mixed gases were purchased from MS Gas Corporation.
Fabrication of precursor dense film membranes
LPSQ and PI were dissolved in THF at 5 wt% concentration, and the dope solution was mixed on a roller overnight. Filtration was performed using
Characterization of the micropore structure of CMS PI-LPSQ dense film membranes
6FDA-DAM:DABA (3:2) was the fluorinated polyimide used as a precursor for the blending approach incorporating ladder structured-poly (phenyl-co-pyridylethyl)silsesquioxane with phenyl:pyridylethyl ratio of 6:4 (LPPyr64), LPSQ to induce thermo-oxidative crosslinking during pyrolysis. The homogeneous blending of PI and LPSQ were achieved by the H-bonding between pyridyl sites in LPSQ and acid sites in PI. It was confirmed by the asymmetric carbonyl peak in the FT-IR spectrum, which exhibited an
Conclusions
In summary, we investigated the effect of thermo-oxidative crosslinking of siloxane in PI/LPSQ CMS membranes on gas separation performance which was overlooked in the previous study. The HF/CHF3 gases, derived from the fluorinated PI, induced the partial etching of SiO2 in the hybrid CMS membranes, enhancing the formation of effective ultramicropores for C3H6/C3H8 separation as seen from the DFT-based pore size distribution. With structural characterization, our thorough transport analyses
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.
Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4007987) and the “Next Generation Carbon Upcycling Project” (Project No.2018M1A2A6075919) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, Republic of Korea.
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These authors contributed equally.