Fluorine-containing polyimide/polysilsesquioxane carbon molecular sieve membranes and techno-economic evaluation thereof for C3H6/C3H8 separation

https://doi.org/10.1016/j.memsci.2019.117660Get rights and content

Highlights

  • Fluorinated polyimide/ladder-like polysilsesquioxane CMS membranes were investigated for C3H6/C3H8 separation.

  • Thermo-oxidative crosslinking and partial etching of LPSQ formed impermeable silica phases.

  • Embedded silica in the carbon matrix enhanced effective ultramicropores for C3H6/C3H8 separation efficiency.

  • CMS membranes showed C3H6/C3H8 selectivity of 67 from enhanced diffusivity selectivity.

  • Techno-economic analyses showed economic feasibility of a membrane-distillation hybrid process.

Abstract

Herein, we investigated the effect of thermo-oxidative crosslinking of siloxanes on the C3H6/C3H8 separation performance of carbon molecular sieve (CMS) membranes derived from polymer blends of a fluorine-containing polyimide and a ladder-structured polysilsesquioxane (PI/LPSQ). The PI/LPSQ precursors self-generated fluorinated gases, which possibly lead to the cleavage of double-stranded siloxanes during pyrolysis. At the same time, residual siloxanes underwent thermo-oxidative crosslinking, which resulted in densification into nonporous inorganic SiO2 phases. Such impermeable SiO2 phases adversely affected the gas diffusion, decreasing C3H6 permeability but contributed to the significant enhancement in the C3H6/C3H8 selectivity up to as much as 67, because of the substantially enhanced diffusivity selectivity. Density functional theory-based pore size distribution analysis exhibited that the narrower pores in the range of 5.0–5.5 Å emerged for the PI/LPSQ (80/20 w/w) CMS, supporting the enhancement in C3H6/C3H8 selectivity. Also, PI/LPSQ (80/20 w/w) CMS fibers aged for 30 days showed C3H6 permeance of 2.9 GPU and a C3H6/C3H8 selectivity of 57. Furthermore, the techno-economic analysis verified the economic feasibility of the proposed membrane for the C3H6/C3H8 separation process. It reflected that higher C3H6 permeability plays a significant role in reducing the total cost of C3H6/C3H8 separation process as long as the C3H6/C3H8 selectivity is above 30, implying the significance of anti-aging in CMS hollow fiber membranes.

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.

References (63)

  • J.H. Shin et al.

    Rigid double-stranded siloxane-induced high-flux carbon molecular sieve hollow fiber membranes for CO2/CH4 separation

    J. Membr. Sci.

    (2019)
  • N. Bhuwania et al.

    Engineering substructure morphology of asymmetric carbon molecular sieve hollow fiber membranes

    Carbon

    (2014)
  • O. Sanyal et al.

    Cause and effects of hyperskin features on carbon molecular sieve (CMS) membranes

    J. Membr. Sci.

    (2018)
  • M.R. Kosuri et al.

    Defect-free asymmetric hollow fiber membranes from Torlon®, a polyamide–imide polymer, for high-pressure CO2 separations

    J. Membr. Sci.

    (2008)
  • M.L. Chng et al.

    Enhanced propylene/propane separation by carbonaceous membrane derived from poly (aryl ether ketone)/2,6-bis(4-azidobenzylidene)-4-methyl-cyclohexanone interpenetrating network

    Carbon

    (2009)
  • M. Kiyono et al.

    Effect of polymer precursors on carbon molecular sieve structure and separation performance properties

    Carbon

    (2010)
  • W. Qiu et al.

    Gas separation performance of 6FDA-based polyimides with different chemical structures

    Polymer

    (2013)
  • S. Park et al.

    Side-chain engineering of ladder-structured polysilsesquioxane membranes for gas separations

    J. Membr. Sci.

    (2016)
  • A.F. Ismail et al.

    Direct measurement of rheologically induced molecular orientation in gas separation hollow fibre membranes and effects on selectivity

    J. Membr. Sci.

    (1997)
  • M. Das et al.

    Performance of 6FDA–6FpDA polyimide for propylene/propane separations

    J. Membr. Sci.

    (2010)
  • R.J. Swaidan et al.

    Enhanced propylene/propane separation by thermal annealing of an intrinsically microporous hydroxyl-functionalized polyimide membrane

    J. Membr. Sci.

    (2015)
  • R.L. Burns et al.

    Defining the challenges for C3H6/C3H8 separation using polymeric membranes

    J. Membr. Sci.

    (2003)
  • S.S. Chan et al.

    C2 and C3 hydrocarbon separations in poly(1,5-naphthalene-2,2′-bis(3,4-phthalic) hexafluoropropane) diimide (6FDA-1,5-NDA) dense membranes

    J. Membr. Sci.

    (2002)
  • J.J. Krol et al.

    Polyimide hollow fiber gas separation membranes: preparation and the suppression of plasticization in propane/propylene environments

    J. Membr. Sci.

    (2001)
  • C. Staudt-Bickel et al.

    Olefin/paraffin gas separations with 6FDA-based polyimide membranes

    J. Membr. Sci.

    (2000)
  • K. Tanaka et al.

    Permeation and separation properties of polyimide membranes to olefins and paraffins

    J. Membr. Sci.

    (1996)
  • M.N. Islam et al.

    Preparation and gas separation performance of flexible pyrolytic membranes by low-temperature pyrolysis of sulfonated polyimides

    J. Membr. Sci.

    (2005)
  • C. Karunaweera et al.

    Fabrication and characterization of aging resistant carbon molecular sieve membranes for C3 separation using high molecular weight crosslinkable polyimide, 6FDA-DABA

    J. Membr. Sci.

    (2019)
  • R.J. Swaidan et al.

    Spirobisindane-based polyimide as efficient precursor of thermally-rearranged and carbon molecular sieve membranes for enhanced propylene/propane separation

    J. Membr. Sci.

    (2016)
  • M. Kanezashi et al.

    Separation of propylene/propane binary mixtures by bis(triethoxysilyl) methane (BTESM)-derived silica membranes fabricated at different calcination temperatures

    J. Membr. Sci.

    (2012)
  • M. Binns et al.

    Strategies for the simulation of multi-component hollow fibre multi-stage membrane gas separation systems

    J. Membr. Sci.

    (2016)
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