Cardo-type porous organic nanospheres: Tailoring interfacial compatibility in thermally rearranged mixed matrix membranes for improved hydrogen purification

Highlights

• TR-PBO/TC-cPSB MMMs were prepared for H₂ purification from the H₂/CO₂ mixture.

• The MMMs simultaneously increased gas permeability and selectivity due to good interfacial compatibility between TC-cPSB and TR-PBO.

• TR-PBO/TC-cPSB-15 presented separation performance with 312.5 barrer of H₂ permeability and 5.35 of H₂/CO₂ selectivity.

Abstract

Mixed-matrix membrane (MMM) is an effective way to overcome trade-off limitations of conventional polymeric membranes. However, the existence of defect voids at the polymer/filler interface often limits their performance improvement. Similar issues are also present in thermally rearranged polybenzoxazole (TR-PBO)-derived MMMs. To address this challenge, the selection of fillers is of great importance. Herein, a novel organic porous nanosphere (TC-cPSB), which is prepared by the polycondensation of 9,9-bis(4-aminophenyl) fluorene (BAFL) and terephthalaldehyde (TPAL) followed by thermal crosslinking, is chosen to engineer the polymer/filler interface. Benefiting from strong intermolecular interaction (π-π stacking and hydrogen bonding), the TC-cPSB nanosphere can well disperse in TR-PBO matrix with a defect-free interface. With an increase in TC-cPSB loading, well-designed MMMs exhibit a significant “anti-trade-off” phenomenon whereby gas permeability and selectivity increase simultaneously, following the trend predicted by the Maxwell model. Compared with TR-PBO membrane, the MMM containing 15 wt% of nanosphere shows an increase of 282% and 217.6% in H₂/CO₂ selectivity and H₂ permeability, respectively, which is far beyond 2008 Robeson upper bound.

Introduction

Since the beginning of the 21st century, the depletion of petroleum, coal and other fossil fuels has significantly increased the need for new energy resources. Among various energy alternatives, hydrogen is recognised as the one of the future energy carriers due to its unique features such as high heat of combustion, low emissions, environmental friendliness and elemental abundance [1]. As hydrogen is not naturally obtainable, more than 85% of hydrogen production is currently achieved through steam-methane reforming (SMR) coupled with a water-gas shift (WGS) strategy, in which the main by-product is CO₂. Therefore, it is very important to achieve H₂ purification from H₂/CO₂ mixtures for any application (e.g. ammonia production) [2,3]. Compared with traditional H₂ purification techniques such as amine-based absorption, pressure swing adsorption and cryogenic distillation, membrane separation technology have been identified as a promising method due to its low energy consumption, mechanical simplicity, ease of scale up, and a smaller footprint [4,5]. Conventional polymeric membranes such as polyimide (PI) [[6], [7], [8]], polybenzimidazole (PBI) [[9], [10], [11], [12]] and thermally rearranged (TR) polymer [1,[13], [14], [15], [16], [17], [18]] occupy a major market share because of their low cost, well processability and ease of scale up. However, most of polymeric membranes usually suffer from a trade-off between permeability and selectivity, i.e., polymers with high gas permeability generally have low gas selectivity and vice versa [[19], [20], [21], [22]].

Mixed matrix membranes (MMMs), which contain dispersed inorganic filler and a continuous polymer phase, have been recognised as a suitable alternative to overcome the deficiencies of the trade-off effect because of the combination of the easy processibility of polymer and the separation performance of the inorganic filler [19,[23], [24], [25], [26]]. As H₂ has a smaller kinetic diameter (2.9 Å) and is less condensable than CO₂ (3.3 Å), the MMMs should have a favourable H₂/CO₂ diffusivity selectivity but unfavourable H₂/CO₂ solubility selectivity due to the solution-diffusion model. Therefore, the majority of selected fillers are porous materials that have a strong size-sieving ability, e.g., carbon molecular sieves (CMS) [27], zeolites [28,29], covalent organic frameworks (COFs) [30] and metal-organic frameworks (MOFs) [8,9,[31], [32], [33], [34], [35], [36]]. They can increase the H₂/CO₂ diffusivity selectivity, and consequently improve the H₂/CO₂ separation performance of MMMs. Despite the great advances made in the field of MMMs, the results are still far from satisfactory. The major bottleneck is that such MMMs suffer from poor compatibility between polymers and fillers, which make it difficult to obtain homogeneous filler dispersions without agglomerates. In this scenario, some voids or defects occur at the polymer-filler interface, forming non-selective pathways for gas molecules, which ultimately reduces their size-sieving ability [2,4,19,37]. Similar issues are also present in thermally rearranged polybenzoxazole (TR-PBO)-derived MMMs [23,[38], [39], [40]]. For instance, after the incorporation of multi-walled carbon nanotubes (MWCNTs) or porous aromatic framework (PAF-1) into thermally rearranged polybenzoxazole-co-imide (TR-PBOI), a trade-off phenomenon between H₂ permeability and H₂/CO₂ selectivity was observed [38,39]. To achieve good interfacial compatibility, a wise selection of the filler/polymer pair should be considered first, because this determines the interfacial interaction between the filler phase and polymer phase, as well as the dispersion of filler in the polymer matrix. Therefore, the continued exploration of new porous filler to form a suitable filler/TR-PBO pair is of great significance for developing TR-PBO-derived MMMs with both high H₂ permeability and good H₂/CO₂ selectivity.

With this necessity in mind, in this work, a novel porous organic nanosphere (TC-cPSB) was synthesised by the polycondensation of 9,9-bis(4-aminophenyl) fluorene (BAFL) and terephthalaldehyde, followed by thermal crosslinking. It was then used as porous filler to form the TR-PBO/TC-cPSB pair for the corresponding MMMs fabrication with the targeted interfacial interaction. This novel TC-cPSB nanosphere has three unique features. First, the inherent porous structure of TC-cPSB nanosphere can form additional channels for the gas transport through the membrane, which is anticipated to enhance the H₂ permeability of MMMs. Second, the thermal crosslinking temperature (350 °C) of TC-cPSB nanosphere is the same as that of the thermal rearrangement temperature of TR-PBO, which is useful for maintaining the structural stability of TC-cPSB nanosphere in MMMs, and this ultimately improves the overall stability of MMMs. Finally, strong interactions (e.g. hydrogen bonds and π-π stacking of benzene rings) exist between TC-cPSB and TR-PBO, which can enhance the interphase adhesion to eliminate incompatibility. As excepted, the resultant MMMs exhibit an excellent anti-trade-off phenomenon, namely simultaneous increase in the H₂ permeability and H₂/CO₂ selectivity with the increase in TC-cPSB content. In particular, for 15 wt % of TC-cPSB content, the H₂ permeability and H₂/CO₂ selectivity increase by 300% and 380%, respectively, compared with pure TR-PBO membrane. Clearly, the overall separation performance of H₂/CO₂ transcends the 2008 Robeson upper bound. In short, from both fundamental research and industrial application points of view, this novel MMM based on TC-cPSB has great importance in H₂/CO₂ separation.