Elsevier

Journal of Membrane Science

Volume 474, 15 January 2015, Pages 122-131
Journal of Membrane Science

Gas sorption and transport in thermally rearranged polybenzoxazole membranes derived from polyhydroxylamides

https://doi.org/10.1016/j.memsci.2014.09.051Get rights and content

Highlights

  • TR polymers derived from PHAs were investigated for H2/CO2 separation.

  • TR process allows a great increase in diffusivity and small increase in solubility.

  • Gas permeability increased and solubility decreased at the elevated temperature.

  • H2/CO2 separation performance was improved at the elevated temperature.

Abstract

The sub-nano-sized microcavities in microporous thermally rearranged (TR) polymers can be tuned by varying the conditions of thermal rearrangement. Relatively small cavities were formed by thermal rearrangement of poly(o-hydroxylamide) (PHA) precursors compared to the cavities formed by that of polyimide precursors. TR polymers derived from PHAs, so-called TR-β-polymers, are known to exhibit a well-tuned cavity structure that can be used for H2/CO2 separation. According to a solution-diffusion model, both the permeability and selectivity for H2/CO2 separation were improved at elevated temperatures due to a significant increase in H2 diffusion and a decrease in CO2 sorption. In this study, gas solubility and permeability of five representative small gas molecules (H2, N2, O2, CH4, and CO2) through TR-β-polymer membranes were characterized between 20 °C and 65 °C for gas solubility measurement and between 35 °C and 300 °C for gas permeability measurement. These measurements allowed for the calculation of thermodynamic factors such as the activation energy and heat of sorption.

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.

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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]:P=DS

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:αi/j=PiPj=DiDjSiSj

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).

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