Modelling of Gas Separation in Thermally Rearranged Polymeric Membranes

ELENA TOCCI, ALESSIO CARAVELLA, CARMEN RIZZUTO, GIUSEPPE BARBIERI, YOUNG MOO LEE AND ENRICO DRIOLI

1.1 Introduction

The sustainable development of chemical and related process-oriented industries critically depends on the development of new innovative processes that use materials and energy more efficiently.

The development of advanced membrane technologies and the industrial application of polymeric membranes require good understanding of the materials properties and their transport mechanisms, as well as the realisation of innovative functional materials with enhanced properties. Those different aspects have been discussed in detail in previous lectures and have been recently reviewed in the literature.

Membrane separation technologies have profited from the progress in materials research and processing for device technologies. In particular, membranes for gas separation made mainly of polymer materials, owing to their easy processability and good mechanical properties, compete with other separation processes such as cryogenic distillation and adsorption due to their easy operational handling, relatively small size, low energy consumption, and space efficiency.

Breakthroughs in the development of highly permeable materials for membranes are essential. In this context, microporous polymers are considered efficient membrane materials and good candidates to overcome the well-known Robeson's upper bound. Specific tailoring of the molecular structure can be regarded as a viable approach to obtain improvements on membrane permselectivity due to (i) the loss of inter-segmental packing with a simultaneous inhibition of the intra-segmental (backbone) mobility, and (ii) the weakening of inter-chain interactions (reduction of charge transfer complexes).

Recent progress in the development of microporous polymers as gas separation membranes has been achieved by improving the rigidity of the entire polymer structure to improve the separation performance, since a rigid polymer structure enhances the separation properties and durability of the membranes used for gas separation and storage materials.

Thermally rearranged (TR) polymers are an example of microporous polymers with high permeability and selectivity for the separation of gas mixtures. In particular, they have shown outstanding molecular and ionic transport, as well as separation performance, beyond the limits of more conventional polymers.

An example of process design using TR polymer membranes can be found in the work of Dong et al. from 2015.

1.1.1 Thermally Rearranged (TR) Polymers

TR-polybenzoxazole (PBO) polymers are examples of novel membrane materials with high free volume elements and narrow cavity size distribution based on rigid microporous structures. TR-PBO polymers are glassy aromatic polymers with heterocyclic rings prepared by an in situ thermal treatment (350–450 °C) of hydroxylpolyimide (HPI) precursors with functional groups at the ortho-positions. Since 2007, Lee's group has been studying the thermal conversion mechanism of TR polymers for their application as membrane materials. A hydroxyl-polyimide is prepared by a conventional polycondensation reaction of dianhydrides and diamines with hydroxyl functional groups, obtaining a hydroxyl-containing poly(amic acid) (HPAAc) (Figure 1.1). Then, the HPAAc is converted to hydroxyl polyimide by various imidisation methods, such as thermal, chemical, and azeotropic imidisation, based on the dehydration of the poly(amic acid) structure. The final thermal rearrangement of the hydroxyl polyimide into TR-PBO is carried out at a temperature of 350–450 °C under an inert atmosphere after membrane formation.

Another strategy used in the thermal process is the introduction of thermally labile molecules in a cross-linkable polyimide to prepare highly permeable polyimide membranes by thermal decomposition of the labile units in the solid state. Furthermore, TR copolymers have also been investigated in terms of the concerted effects of different TR polymers with several glassy polymers. These polymers show outstanding physical properties and high permeability, exceeding the limits of more conventional polymers due to their unusual microstructure, a phenomenon that has been explained as the result of the modifications in the polyimide chain during rearrangement into the solid state structure. Such a process leads to the formation of rigid rods with a concomitant conformation randomisation resulting from the formation of meta- and para-linked chains.

Moreover, this causes an increase of the free volume distribution, which improves their general mass transport performance. In fact, during thermal rearrangement into the solid state, a microporous structure with interconnected microcavities is obtained with a distribution of narrow cavities accessible to small gas molecules.

However, it is still challenging to demonstrate how and to what extent the thermal treatment affects the polymer structure at the atomistic and molecular levels, specifically in terms of its configuration, conformation, glassy transition temperature, and/or free volume. In particular, if a chemical reaction occurs during the thermal treatment, the structure–property relationships of the starting and final structures become more and more complex.

The physical properties of TR polymer membranes depend on the polymer backbone structure, as well as on the imidisation method. A great advantage of TR polymers lies in the possibility of determining their cavity size by designing appropriate polymer structures and thermal reaction mechanisms.

1.1.2 Computational Approach to Polymeric Membranes: From Macro- to Atomistic Scale

The design and optimisation of polymeric membranes for gas separation by numerical simulation...