Organic solvent nanofiltration membranes based on polymers of intrinsic microporosity

Highlights

• PIMs with outstanding performance can be developed for advanced OSN membranes.

• Crosslinked PIM-based membranes enhance stability and enable control of membrane properties.

• The molecular weight cut-off for organic solvent nanofiltration membranes is 200–800 g mol⁻¹.

• Investigations on physical aging, solvent resistance, and varying porosity are required.

• Thermal annealing and carbonization of PIM membranes may lead to high solvent and swelling resistance.

High-performance polymer membranes have attracted attention as excellent candidates for the fabrication of separation materials owing to their high versatility and ease of processability. Membranes can be prepared from polymers through solvent evaporation, phase inversion, coating, or interfacial polymerization. Organic solvent nanofiltration (OSN) processes require membranes with high solvent resistance, excellent thermal and chemical stability, and high separation performance. Polymer membranes have shown great potential in OSN for separating valuable pharmaceutical compounds, dyes, and other organic molecules within a molecular weight range of 100–2000 g mol⁻¹ in organic solvents. Polymers of intrinsic microporosity (PIMs) have been used to fabricate OSN membranes with outstanding thermal and chemical stability in addition to high intrinsic porosity. In this study, we discuss different design aspects that should be considered for achieving high performance in advanced PIM-based OSN membranes operating under harsh conditions.

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.