Organic solvent nanofiltration membranes based on polymers of intrinsic microporosity
Graphical abstract
Introduction
Separation processes are critical in the pharmaceutical and chemical industries, accounting for 40%–70% of capital and operating costs [1]. Pharmaceutical products and fine chemical compounds are often prepared in organic solvents. The reaction mixture comprises valuable products, additives, and impurities that should be separated or removed from the organic solvent and recovered. Membrane technology has provided practical alternatives to replace conventional separation and recovery processes such as evaporation, distillation, extraction, and chromatography. Organic solvent nanofiltration (OSN) has emerged as an energy-efficient membrane separation process to aid in the production of fine chemicals (e.g. pharmaceuticals) and petrochemicals (e.g. diluents), through the separation of solutes with a molecular weight of 100–2000 g mol−1 in organic media [1]. Solvent-resistant membranes with high thermal and chemical stability are required to ensure that separation characteristics are maintained; however, separations are performed in an extensive range of solvents under harsh conditions (including high pressure and temperature) [2]. Thus, there is a critical need in OSN for new membrane materials that possess high separation performance, notable selectivity, and excellent chemical and thermal stability [3••].
Polymer membranes have attracted attention due to their high flexibility and modifiable structure, which can be tailored to fine-tune the separation performance and ease of preparation relative to ceramic and metal membranes [4]. Polymer membranes can be fabricated as flat sheets or hollow fibers. Flat-sheet membranes are usually prepared as (i) a dense film by solvent evaporation, (ii) an integrally skinned asymmetric membrane by phase inversion, or (iii) a thin-film composite membrane by interfacial polymerization [5, 6, 7]. Polymer membranes tend to dissolve or swell in organic solvents; therefore, these unstable membranes need to be crosslinked to ensure membrane stability and solvent resistance under a given pressure and temperature [8•]. However, crosslinking has been found to affect the overall separation performance, and in most cases, crosslinked membranes are tighter than the corresponding noncrosslinked pristine membranes [8•].
In the last decade, polymers of intrinsic microporosity (PIMs) have been investigated for a wide range of applications, including gas separation [9], catalysis [10], quantum dot applications [11••], and OSN [12], owing to their high thermal and chemical stabilities, solution processability, and porosity. PIMs are prepared from fully fused-ring macromolecule chains or from contorted bridged bicyclic or spirocyclic units that exist in the polymer backbone. The presence of fused-rings or contorted units enhance the backbone polymer rigidity, which frustrate chain packing, and prohibit bond rotation along the polymer backbone [13]. The loss of rotation and insufficient packing of polymer chains allow PIMs to gain rigidity and contorted molecular configurations. As a result, a large number of pores are generated along with a high fractional free volume, which leads to a high Brunauer–Emmett–Teller (BET) surface area of 200–1000 m2 g−1. The porosity and free volume of PIMs can be easily tailored by changing the kinked structure or introducing functional groups to the main backbone [14,15]. Recent studies have shown that PIMs may be an excellent candidate for OSN applications [16,17]. However, previous work on PIMs in OSN is limited to a few research articles and patents; thus, further exploration is needed. Herein, we provide our perspective with a focus on exploring and incorporating PIMs in OSN membranes and describing current needs and challenges to further improve OSN membrane fabrication and performance.
Section snippets
PIM-based OSN membranes
Based on their structural connectivity, PIMs have been classified as linear, ladder, or network polymers [15]. In general, linear PIMs are made from contorted units connected through single bonds with strictly hindered rotation. Examples of linear PIMs include polyimides, polyacetylenes and polybenzimidazole [15]. However, ladder PIMs contain two bonds between monomeric units that are highly rigid and restricted to rotation, such as PIM-1 and PIM-7 [15]. Most linear and ladder PIMs are
Crosslinking
Separation by OSN membranes must be performed in harsh solutions such as organic solvents or in the presence of acids and bases. Polymer membranes, including PIM-based OSN membranes, have been prepared from solution-processable polymers using organic solvents. Over the past decade, several crosslinking techniques have been developed to improve membrane stability for various OSN-related applications. Crosslinked membranes prepared from PBI [2], polyimide [27], polyethersulfone [28],
Physical aging
In general, physical aging is a significant obstacle for polymeric membranes. Physical aging is defined as reversible densification and rearrangement of polymer chains driven by the dissipation of non-equilibrium free volume toward an equilibrium state to minimize the free space generated between polymer chains [36]. The mechanism of physical aging has been thermodynamically described by Struik [37], who reported that the degree and rate of aging are proportional to the difference between the
Porosity
The porosity of polymers and membranes is a vital factor that significantly affects the separation process. The separation mechanism is usually governed by two essential parameters: solubility and diffusivity [41]. Solubility is affected by the solute type and its interaction with the polymer chains, whereas diffusivity is related to the potential of the solutes to travel across a membrane [42]. Porous membranes tend to have a higher diffusivity than tight, nonporous membranes [43]. The
Carbonization
Carbon molecular sieve (CMS) membranes have recently attracted attention for membrane-based gas separation and are expected to be utilized in liquid separations. These membranes have demonstrated a notable enhancement in separation performance compared with pristine membranes [48]. Relative to conventional low-free-volume glassy polymer precursor materials, CMS membranes prepared from PIMs are exceptional, as intrinsic microporosity exists in their original structure before pyrolysis [49]. The
Conclusion and outlook
Here, we have outlined viable paths by which PIMs can be developed for advanced OSN membranes with outstanding performance. The involvement of PIMs in OSN will enable the production of membranes with high flux and selectivity; however, it will present the challenge of solvent resistance. Membrane stability can be enhanced via crosslinking by selecting proper crosslinkers that provide membrane stability without dramatically affecting membrane performance. Moreover, the membrane performance will
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
We thank Gergo Ignacz and Martin Gede for useful discussions on the manuscript. This study was funded by the King Abdullah University of Science and Technology (KAUST).
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