Gas sorption and transport in thermally rearranged polybenzoxazole membranes derived from polyhydroxylamides
Graphical abstract
(a) Polymer structure of precursors and TR-β-PBOs. (b) Solubility coefficients of precursors and TR-β-polymers as a function of critical temperature of small gas molecules measured at 1 bar, 35 °C.
Introduction
Microporous materials have been investigated for gas storage, separation, and catalysis due to their high porosity and surface area. The utilization of microporous polymers as gas separation membrane materials has occurred in various applications such as ammonia purge gas recovery, natural gas stripping, and CO2 capture [1], [2], [3], [4]. Recently reported microporous polymers with high free volumes are distinguished from conventional polymers, which typically have a well-packed and non-porous structure. Rigid and contorted chain structures in microporous polymers result in higher free volume elements with effective size sieving properties that can be used as gas separation membranes. There has been recent progress in new types of polymers such as polymers of intrinsic microporosity (PIMs) and thermally rearranged (TR) polymers [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. These have demonstrated extraordinary gas permeability with relevant selectivity. Moreover, easy scale-up and fabrication of highly permeable polymers are important for large-scale industrial processes, especially for CO2 separation in carbon capture and sequestration (CCS) for post-combustion and pre-combustion to battle global warming issues, as well as natural gas sweetening applications [8].
The major emission sources of CO2 are coal-fired power plants as a result of fuel combustion (post-combustion, CO2/N2 separation) and the water gas shift (WGS) reaction in the integrated gasification combined cycle (IGCC) process (pre-combustion, H2/CO2 separation). Pre-combustion CO2 separation processes are usually operated at high pressure (>10 bar) and high temperature (150–900 °C) [15], [16], [17]. The pressure-driven gas separation membrane process has the benefit of high flux during the high pressure pre-combustion CO2 separation process. However, the use of membrane materials is quite limited because the operation conditions are harsh and include high temperatures and pressures; commercially available membranes utilize polymers with low thermal stability [18], [19], [20]. Depending on the operation conditions of the WGS reaction, the feed gas is usually at a temperature higher than 300 °C, where common polymeric materials cannot be used because they undergo rapid degradation. As a result, various palladium-based metals and ceramics have been studied for pre-combustion CO2 separation membranes; however, these are difficult to apply to large-scale operations due to problems in membrane module fabrication [16], [17]. A heterocyclic polybenzimidazole (PBI), as a high-temperature polymer, has been investigated for use in pre-combustion CO2 separation membranes because of its excellent thermal, chemical, and mechanical resistances [21], [22], [23]. Polyimides (PI) and poly(ether ether ketone) (PEEK) have also been investigated due to their high thermal stability, but the operation conditions of pre-combustion CO2 separation are still too harsh for these materials. A cooling step is necessary in order to utilize commercial PI and PEEK membranes for pre-combustion CO2 separation [24], [25], [26]. Alternatively, thermally rearranged (TR) polymer membranes are a good candidate for high temperature CO2 separation due to their high thermal stability and rigid polymer structures [27].
Thermal rearrangement is a suitable method to produce polymer membranes with high gas permeability and selectivity. One of the advantages of TR polymers is that the cavity sizes and distributions can be controlled by adopting specific polymer structures or thermal conversion routes and/or conditions [6]. To improve H2/CO2 selectivity, TR polymers obtained from poly(o-hydroxylamide)s (PHAs), so-called TR-β-polymers, were investigated because they present relatively smaller cavity sizes that can be tuned for H2/CO2 separation [27]. TR-β-polymers are compared with TR polybenzoxazole (TR-PBO) from hydroxyl polyimide, so-called TR-α-polymer for their distinguished physical properties such as free volume or gas transport properties. TR-β-polymers exhibit lower thermal conversion temperatures around 350 °C with the evolution of H2O molecules, which is advantageous for economic membrane preparation [27]. The high thermal stability (stable above 300 °C) allows TR polymers to be applied to pre-combustion CO2 separation processes in order to produce H2 and generate power [27].
In this study, the gas sorption, diffusion, and permeation properties of TR-β-polymer membranes were investigated in order to characterize their gas transport behaviors. Gas permeability and solubility values of five representative small gas molecules including H2, N2, O2, CH4, and CO2 were characterized for TR-β-polymer membranes. Gaining information about gas solubility can allow for the study of the individual contributions of gas permeability, diffusivity, and solubility of TR-β-polymer membranes according to the solution–diffusion model [28], [29], [30]. We also investigated the effect of operation temperature on gas permeability and solubility. It was our aim to investigate the thermodynamic aspects of gas permeation and sorption measurements at elevated temperatures, including the activation energies and heat of sorption [17], [31], [32].
Section snippets
Background
Gas transport through a polymer membrane is explained by a solution–diffusion model where the gas permeability coefficient (P) is a product of the diffusion coefficient (D) and the sorption coefficient (S), as described in the following equation [31], [33]:
The selectivity (α) is related to the separation properties of the membranes and is calculated as the ratio between the permeabilities of penetrants, as described in the following equation:
Sorption, as a thermodynamic
Polymer preparation
Two rigid aromatic acid chlorides of meta-phenylene and para-phenylene, i.e., isophthaloyl dichloride (IPCl) and tetraphthaloyl dichloride (TPCl) were obtained from Aldrich Chemical Co. (Milwaukee, WI, USA), and 2,2′-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF) as a bisaminophenol was obtained from Central Glass Co., Ltd. (Tokyo, Japan). N-methyl-2-pyrrolidinone (NMP), dimethylformamide (DMF), n-hexane, and toluene were obtained from Aldrich Chemical Co. (Milwaukee, WI, USA) and
Gas sorption
The sorption isotherm curves of glassy polymers usually follow the dual-mode sorption model. TR-β-polymers and their precursor PHAs also exhibited the dual-mode sorption isotherms. Fig. 2 shows the sorption isotherm curves of the precursor polymers and the resulting TR polymers with meta and para linkages, i.e., mPHA, pPHA, TR-mPBO, and TR-pPBO, respectively. The gas solubility of PHAs was improved after thermal rearrangement into TR polymers due to the increased free volume. However, the
Conclusions
Sorption and transport behavior of small gas molecules in TR-β-polymers derived from PHA precursors were studied to investigate the effect of polymer structure (meta vs. para linkage) and temperature. Solubility coefficients of PHAs before and after thermal rearrangement were slightly increased for all small gas penetrants, regardless of meta or para linkages in the polymer chain. Gas permeability and diffusivity of PHAs also increased after thermal rearrangement. However, the increases in gas
Acknowledgments
This research was supported by the Korea Carbon Capture & Sequestration R&D Center (KCRC) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2014M1A8A1049305).
References (46)
- et al.
Polymeric CO2/N2 gas separation membranes for the capture of carbon dioxide from power plant flue gases
J. Membr. Sci.
(2006) - et al.
Energy efficient polymeric gas separation membranes for a sustainable future: a review
Polymer
(2013) - et al.
Thermally rearranged polybenzoxazoles membranes with biphenyl moieties: monomer isomeric effect
J. Membr. Sci.
(2014) - et al.
Recent developments on membranes for post-combustion carbon capture
Curr. Opin. Chem. Eng.
(2011) Polymer membranes for gas separation
Curr. Opin. Solid State Mater. Sci.
(1999)- et al.
High performance polymers for membrane separation
Polymer
(1994) - et al.
Membrane-based gas separation
J. Membr. Sci.
(1993) - et al.
Polybenzimidazole composite membranes for high temperature synthesis gas separations
J. Membr. Sci.
(2012) - et al.
Highly gas permeable and microporous polybenzimidazole membrane by thermal rearrangement
J. Membr. Sci.
(2010) - et al.
In-line formation of chemically cross-linked P84® co-polyimide hollow fibre membranes for H2/CO2 separation
Sep. Purif. Technol.
(2010)
Carbon dioxide recovery using a dual gas turbine IGCC plant
Energy Convers. Manage.
Gas sorption, diffusion, and permeation in thermally rearranged poly(benzoxazole-co-imide) membranes
J. Membr. Sci.
Sorption and transport of small gas molecules in thermally rearranged (TR) polybenzoxazole membranes based on 2, 2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF) and 4, 4′-hexafluoroisopropylidene diphthalic anhydride (6FDA)
J. Membr. Sci.
Gas sorption and characterization of thermally rearranged polyimides based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)
J. Membr. Sci.
The solution-diffusion model: a review
J. Membr. Sci.
Gas sorption and permeation of glassy polymers with microvoids
Prog. Polym. Sci.
Dual sorption theory
J. Membr. Sci.
Mixed gas sorption in glassy polymers: equipment design considerations and preliminary results
J. Membr. Sci.
Thermally rearranged (TR) polymer membranes for CO2 separation
J. Membr. Sci.
Future directions of membrane gas separation technology
Ind. Eng. Chem. Res.
Membrane gas separation: a review/state of the art
Ind. Eng. Chem. Res.
Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage
Chem. Soc. Rev.
Polymers with cavities tuned for fast selective transport of small molecules and ions
Science
Cited by (36)
Rational macromolecular design and strategies to tune the microporosity for high-performance O<inf>2</inf>/N<inf>2</inf> separation membranes
2024, Separation and Purification TechnologySupercritical CO<inf>2</inf> permeation in polymeric films: Design, characterization, and modeling
2023, Materials and DesignState of the art and prospects of chemically and thermally aggressive membrane gas separations: Insights from polymer science
2021, PolymerCitation Excerpt :TR polymers exhibit exceptional thermal stability up to 400 °C [343,362–364], making them attractive membrane materials for gas separations at high temperatures. A series of TR polymer derived from poly(o-hydroxylamide) has been prepared by Lee et al. [64,365]. Pure-gas H2 and CO2 permeability tests at temperatures up to 300 °C showed higher gas permeability and, more importantly, enhanced H2/CO2 selectivity with increasing temperature [64,365], a result of significantly reduced of CO2 solubility at higher temperature [64].
Thermally rearranged semi-interpenetrating polymer network (TR-SIPN) membranes for gas and olefin/paraffin separation
2021, Journal of Membrane Science