Membranes of Polymer of Intrinsic Microporosity PIM-1 for Gas Separation: Modification Strategies and Meta-Analysis

Abstract

Polymers of intrinsic microporosity (PIMs) have received considerable attention for making high-performance membranes for carbon dioxide separation over the last two decades, owing to their highly permeable porous structures. However, challenges regarding its relatively low selectivity, physical aging, and plasticisation impede relevant industrial adoptions for gas separation. To address these issues, several strategies including chain modification, post-modification, blending with other polymers, and the addition of fillers, have been developed and explored. PIM-1 is the most investigated PIMs, and hence here we review the state-of-the-arts of the modification strategies of PIM-1 critically and discuss the progress achieved for addressing the aforementioned challenges via meta-analysis. Additionally, the development of PIM-1-based thin film composite membranes is commented as well, shedding light on their potential in industrial gas separation. We hope that the review can be a timely snapshot of the relevant state-of-the-arts of PIMs guiding future design and optimisation of PIMs-based membranes for enhanced performance towards a higher technology readiness level for practical applications.

1. Introduction

Polymers of intrinsic microporosity (PIMs) refer to a relatively new class of porous materials. The synthesis of these organic nanoporous materials, classified as microporous as their pore diameters are smaller than 2 nm, was first disclosed in a patent in 2003 [1] and later reported in the literature in 2004 [2]. The macromolecular backbone of the PIMs is composed of fused rings that prohibit large-scale conformational changes and incorporates sites of contortion, such as spiro-centres, giving rise to a randomly twisted structure that cannot efficiently fill space in the solid state [1]. Therefore, the fractional free volume in PIMs is high, and free volume elements are effectively interconnected, behaving like micropores. The bottleneck or gates interconnecting micropores behave as sieves for gas molecules with different sizes and shapes. Such interconnected microporosity is analogous to the framework structure of ordered molecular sieves such as zeolites. Additionally, such high free volume endows PIMs with a large accessible internal surface area (700–900 m2 g−1), giving rise to high sorption capacity for many molecules. The high adsorption capacity also contributes towards the fast transport of molecules, considering PIM-1 membranes follow the solution-diffusion model. Consequently, PIMs offer remarkable combinations of permeability and selectivity with higher values than most traditional synthetic polymeric membranes [3–5].

PIMs-based membranes are reported for various gas separation applications, including air separation [6], hydrogen recovery [7, 8], and other more challenging scenarios such as separation of ethylene (C2H4)/ethane (C2H6) and corrosive fluorinated gases [9]. Owing to their appropriate pore size and preferential adsorption towards carbon dioxide (CO2), PIMs membranes have been extensively investigated for selective CO2 separation. In 2005, the first gas permeation data of a prototype PIMs, PIM-1 membrane was reported, revealing a CO2 permeability of 2300 barrer, along with reasonable selectivity of 25 for CO2/nitrogen (N2) [10], which surpassed the upper bound established by Robeson in 1991 [11] and led to the revision of the upper bounds of performance in 2008 [3]. In following years, other PIMs with enhanced separation performance have been developed and have led to two more recent upper bounds [4, 5]. To date, over 600 publications have been published on CO2 separation using PIM-based membranes, with 80% of these published in the last decade. The exceptional high permeability (up to 50,000 barrer for CO2 [5]) and reasonably good selectivity makes PIMs membranes one of the most competitive candidates for their use in carbon capture and storage to achieve the net zero goals set by different governments [12].