Thermally rearranged polybenzoxazoles and poly(benzoxazole-co-imide)s from ortho-hydroxyamine monomers for high performance gas separation membranes
Graphical abstract
Introduction
In recent decades significant research efforts have been made to identify novel polymer materials with an adequate balance of productivity and permselectivity to be considered as candidates for competitive gas separation membranes. Importantly, these materials are useful for a variety of industrial operations, such as stripping of carbon dioxide from natural gas, nitrogen and oxygen enrichment, hydrogen recovery from petrochemical recycling, purging of gas streams, water vapor removal from air, and recovering volatile organic compounds and monomers [1], [2], [3], [4]. Membrane separation technologies are more energy efficient, simple, and lower in cost than classical methods for gas separation and purification.
Commercial applications rely primarily on solution-diffusion membranes due to their ability to facilitate transportation of gas molecules. The specific mechanism of this type of membrane involves molecular scale interactions between the permeating gas and the membrane polymer. Thus, permeability can be expressed in terms of the gas transport and sorption coefficients [5]. A common way of expressing this relationship is as follows:
In this relationship, the quantity S represents the solubility coefficient, which is thermodynamic in nature, and is affected by polymer–penetrant interactions. Conversely, the average diffusion coefficient D is kinetic in nature and is determined primarily by the penetrant molecular size and polymer–penetrant dynamics. The paramount importance of the diffusion coefficient determines the gas separation behavior and defines the existence of a trade-off between gas permeability and gas selectivity [6], [7].
The chemical structure of a polymer greatly affects its properties and behavior as a gas separation membrane. Thus, research on this topic has been targeted at developing polymers with a high fractional free volume (FFV) to improve permeability and high molecular rigidity to attain high selectivity [8], [9]. Indeed, these are the goals of numerous new polymers that have been synthesized in recent years, especially glassy aromatic polymers [10], [11], [12]. In this regard, aromatic polyimides have achieved major importance as they offer affordable synthesis routes and balanced properties that are especially well suited for gas separation, namely, they are thermally stable, consist of glassy materials with a high molecular stiffness and glass transition temperatures commonly over 300 °C, and can be solubilized by organic solvents, thereby simplifying the fabrication of dense membranes by classical casting techniques [13].
One successful example of polyimides with enhanced gas separation properties is thermally rearranged polymers (TR polymers), which are derived from thermal structural conversion of precursor aromatic polyimides with hydroxyl groups in the ortho position to the imide ring [14], [15]. This thermally driven rearrangement, produced in the solid state at high temperature, leads to dramatic changes in the composition and molecular conformation of ortho-hydroxy-containing polyimides, resulting in polybenzoxazoles (TR-PBOs) with an unusual microporous structure that translates into a substantial improvement in gas permeability, with minimal loss of selectivity [16], [17].
Dianhydride hexafluoroisopropylidene diphthalic (6FDA) is the preferred dianhydride monomer to prepare these special polyimides, as it is a strong electrophile that affords soluble polyimides. Aromatic dihydroxydiamines (2HDA), 2,2′-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (APAF), 3,3′-dihydroxybenzidine (pHAB), and its isomer 3,3′-diamino-4,4′-dihydroxybiphenyl (mHAB) have been used for these purposes [16], [17], [18], [19], [20]. APAF is quite expensive; much more so than pHAB, while mHAB is a non-commercial reagent that was recently reported to be a suitable raw material for TR polymers [20]. All three 2HDAs have proven to be good nucleophiles and present a reasonably high reactivity against the dianhydride 6FDA, although the diamine APAF is comparatively less reactive because of the electron withdrawing effect of its hexafluoroisopropylidene group. The combination of lower reactivity and significant price makes the diamine APAF less recommendable. Furthermore, mHAB and pHAB endow the final materials with a better thermal stability.
Within the framework of the continuous research effort on this topic, we introduced in the present study two commercial ortho-hydroxydiamines (HDAs) as HPI monomers. The class of molecules known as ortho-aminophenols is sensitive to both air and light, and can spontaneously evolve to oxidized side products. These oxidized chemicals are inactive with respect to polycondensation, and that is the reason why HDAs are not traditionally used for the synthesis of aromatic polymers, unless they contain an electron acceptor group such as trifluoromethyl or hexafluoroisopropylidene [21]. However, it is possible to achieve high molecular weight polyamides and polyimides by using trimethylsilyl or hydrochloride derivatives [22], [23]. In the present study, we used the latter approach wherein the preparation of HPIs from 2,4-diaminophenol dihydrochloride (DAP-Cl) and 4,6-diaminoresorcinol dihydrochloride (DAR-Cl) was optimized to obtain high molecular weight polymers. TR poly(benzoxazole-co-imide) membranes have been previously reported by Jung et al. [24]; however, these authors did not use HDA as monomers, but rather mixtures of aromatic diamines and 2HDA in combination with dianhydrides to prepare HPIs that were eventually converted into poly(benzoxazole-co-imides) (TR-PBOI) by thermal rearrangement.
In the present study, the polycondensation reaction was refined by using chlorotrimethylsilane as a polyimidization promoter and by thoroughly purifying DAP-Cl, DAR-Cl, and the dianhydride 6FDA. In this way, high molecular weight HPIs with good film-forming capability could be attained. For comparison purposes, a pristine polyimide was also prepared from 6FDA and meta-phenylenediamine, which allowed for the creation of three chemically related polymers with final compositions corresponding to polyimide (PI), poly(benzoxazole-co-imide) (PBOI) and polybenzoxazole (PBO).
All of the precursor polyimide films and final TR polymer films were systematically characterized by spectroscopic methods. Special emphasis was given to monitoring the thermal rearrangement process, which was followed by thermogravimetry, X-ray diffraction, and free volume analysis. In addition, the gas permeation properties of the final membranes were evaluated by measuring single gas permeability properties and ideal selectivity.
Section snippets
Materials
Chlorotrimethylsilane (CTMS), pyridine (Py), 4-dimethylaminopyridine (DMAP), o-xylene, acetic anhydride, anhydrous N,N-dimethylacetamide (DMAc) and anhydrous N-methyl-2-pyrrolidinone (NMP) were purchased from Aldrich and used as received. 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) was provided by Cymit Química (Barcelona) and sublimed just before use. Diamines including 2,4-diaminophenol dihydrochloride (DAP), 4,6-diaminoresorcinol dihydrochloride (DAR) and
Synthesis and characterization of PI-MPD, HPI-DAP and HPI-DAR
Polyimide PI-MPD was synthesized by a classical and quantitative two-step procedure in which diamine MPD was combined with dianhydride 6FDA to polymerize at room temperature in a solution of DMAc. The intermediate poly(amic acid) was attained through a base-assisted in situ silylation method [28] which involves the use of chlorotrimethylsilane as the silylating agent and pyridine and 4-dimethylaminopyridine as activating reagents. Next, the intermediate poly(amic acid) was chemically imidized
Conclusions
Two high molecular weight polyimides (HPIs) having ortho-hydroxyl groups to the nitrogen of the imide moiety were attained via the reaction of 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and the hydrochloride salts of two inexpensive monomers, 2,4-diaminophenol (DAP) and 4,6-diaminoresorcinol (DAR), using an in situ silylation method. Both polyimides were processed in the form of films with good mechanical properties. Note that although TR materials TR-PBOs do not
Acknowledgements
The authors would like to thank the Ministry of Economy and Competitiveness (MINECO) for the financial support of this work within the framework of the Plan Nacional de I+D+i through the research projects; MAT2011-25513, MAT2010-20668 and MAT2013-45071-R. This research was also 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 (NRF-2014M1A8A1049305). The
References (34)
- et al.
The solution-diffusion model: a review
J. Membr. Sci.
(1995) Correlation of separation factor versus permeability for polymeric membranes
J. Membr. Sci.
(1991)The upper bound revisited
J. Membr. Sci.
(2008)- et al.
Rigid and microporous polymers for gas separation membranes
Prog. Polym. Sci.
(2015) - et al.
Gas separation properties of systems with different amounts of long poly(ethylene oxide) segments for mixtures including carbon dioxide
Int. J. Greenhouse Gas Control
(2013) - et al.
Design of gas separation membranes derived of rigid aromatic polyimides. 1. Polymers from diamines containing di-tert-butyl side groups
J. Membr. Sci.
(2010) - et al.
Unexpected thermal conversion of hydroxy-containing polyimides to polybenzoxazoles
Polymer
(1999) - et al.
Thermally rearranged (TR) polymer membranes for CO2 separation
J. Membr. Sci.
(2010) - et al.
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., 415–
(2012) - et al.
Gas permeability, diffusivity, and free volume of thermally rearranged polymers based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)
J. Membr. Sci., 409–
(2012)
Thermally rearranged polybenzoxazoles membranes with biphenyl moieties: Monomer isomeric effect
J. Membr. Sci.
Synthesis and characterization of fluorinated polybenzoxazoles via solution cyclization techniques
Polymer
Highly permeable and selective poly(benzoxazole-co-imide) membranes for gas separation
J. Membr. Sci.
The relationship between the chemical structure and thermal conversion temperatures of thermally rearranged (TR) polymers
Polymer
Cavity size, sorption and transport characteristics of thermally rearranged (TR) polymers
Polymer
Highly gas permeable and microporous polybenzimidazole membrane by thermal rearrangement
J. Membr. Sci.
Gas and liquid separations using membranes: An overview, in: Advanced materials for membrane separations
Cited by (34)
Gas separation performance of solid-state in-situ thermally crosslinked 6FDA-based polyimides
2022, Journal of Membrane ScienceCitation Excerpt :The resulting PBOs showed significantly enhanced gas permeabilities coupled with commensurate decrease in gas-pair selectivities [28]. Several reports revealed that 6FDA-based PBOs made by TR were insoluble in strong solvents, which was ascribed to thermally induced crosslinking; however, evidence of specific groups in the pristine polyimides involved in the crosslinking process was not clearly identified [28–30]. Only a few studies investigated the changes in gas transport properties of non-functionalized polyimides after thermal treatment.
Polymers of intrinsic microporosity and thermally rearranged polymer membranes for highly efficient gas separation
2022, Separation and Purification TechnologyCitation Excerpt :Thermal rearrangement of the polyimide chain into a rigid-rod polymer, yields a bimodal distribution of micropores in the range of 0.3–0.4 nm and 0.7–0.9 nm, which leads to TR-polymers with outstanding selective transport behaviors for small gas pairs. Moreover, the size and distribution of micropores in these polymers, and thus the related separation performances, can be tailored by controlling the film preparation conditions in terms of treatment time and temperature [56,63-67]. In general, PBI membranes have a highly rigid structure, and they show tight chain packing because of the strong intermolecular interactions [62].
Preparation and gas separation performance of thermally rearranged poly(benzoxazole-co-amide) (TR-PBOA) hollow fiber membranes deriving from polyamides
2021, Separation and Purification TechnologyGas transport properties of mixed matrix membranes based on thermally rearranged poly(hydroxyimide)s filled with inorganic porous particles
2020, Separation and Purification Technology