Mixed matrix membranes for gas separation

Abstract

Mixed matrix membranes (MMMs) are hybrid membranes, which have been intensively studied and expected to overcome the drawbacks of both polymeric and inorganic counterparts. In fact, MMMs are still facing great challenges, mostly due to the poor compatibility and adhesion between the fillers and polymer matrix, which considerably reduce MMMs separation performance. To address that issue, the work in this thesis focus on modification methods in order to improve the interfacial adhesion between polymer/fillers in the MMMs and consequently enhance the gas separation efficiency of the MMMs.In the first part of experiment, a non-porous nano-size filler, nanodiamond (ND) was introduced into Pebax copolymer to fabricate the MMM. While being promising filler, the non-porous structure and susceptible to agglomeration of ND are still the issues in gas separation membrane. This chapter proposes an efficient approach as grafting polyethyleneimine (PEI) onto the surface of ND before embedding into the polymer matrix to fabricate the MMMs for CO2/N2 separation.  The presence of PEI layer on ND surface significantly improved the interfacial adhesion and dispersion of ND in the Pebax matrix, which were clearly indicated by SEM and FIB-SEM observation. The improvement of interfacial interaction led to the increment in CO2/N2 selectivity compared to the pristine polymer membranes and the non-PEI MMMs as well. The CO2/N2 selectivity of the Pebax/oxND-PEI 0.5 wt.% increased 25% compared to the neat polymer and 43.66% compared to the Pebax/oxND. This chapter has contributed to a simple but effective method to improve the dispersion of the non-porous nanofiller, as well as enhance the gas separation performance of the MMMs.The next chapter studied the effects of different morphologies of filler on the dispersion, interfacial interaction and gas separation performance of the MMMs. Three types of filler morphologies: conventional polyhedral (P-ZIF), nanorod (R-ZIF) and leaf-shaped nanosheet (L-ZIF) were introduced and investigated. The change in morphology can alter the interfacial interaction between polymer and fillers due to the different aspect ratio and surface structure. The L-ZIF and R-ZIF showed better compatibility with the 6FDA-durene polymer matrix compared to the polyhedral ZIF. L-ZIF improved the gas selectivity of CO2/N2 (30.3%), CO2/CH4 (40%) compared to the neat polymer, while the R-ZIF enhance the CO2 permeability (41%) with comparable gas selectivity to the neat polymer. This chapter's results suggested that the nanorod and nanosheet morphologies are more effective in enhancing the interfacial adhesion between polymer/filler and contributed to the guidance in filler morphology selection to achieve improved gas separation performance.In the following chapter, ZIF nanorod (R-ZIF) was further investigated as the filler and was coated with two types of ILs  before incorporated in the the 6FDA-durene matrix. In the previous chapter, while showing compatibility with the 6FDA-durene matrix at low filler loading (10 wt.%), R-ZIF still formed aggregates in the membrane at high loading (20 wt.%) which decrease the gas separation performance of the MMMs. The ionic liquid decoration improved the interfacial interaction between R-ZIF and the polymer matrix leading to the enhancement in gas separation performance of the PR/IL MMMs which was intensively investigated by conventional SEM, FTIR, single and mix gas tests. The most significant improvements were the increment of 50% in CO2/CH4 selectivity, while maintaining the CO2 permeability of the 10 wt.% R-ZIF/IL MMM. The improvement in gas separation efficiency of the IL-incorporated MMMs compared to the non-IL MMMs was still observed even at high loading of filler (20 wt.%).  The contribution in this part is to confirm that IL-decoration is an effective approach to enhance the interfacial issues and improve the gas separation efficiency of the MMMs.In the last experiment section, micron size polyhedral shape ZIF (P-ZIF) was coated with 3 different ILs and dispersed in 6FDA-durene matrix to prepare the MMMs. As investigated in previous experiment section, P-ZIF exhibited the worst interfacial interaction with the polymer matrix among 3 different morphologies. Thus, it is more challenging to obtain excellent filler/polymer contact between micron-sized P-ZIF and polymer matrix and achieve improvement in gas separation efficiency. Acting as the interfacial binder, IL layer has effectively reduced the non-selective interfacial voids in the MMM and enhanced the polymer/P-ZIF adhesion. The vol.% of interfacial voids of the pristine PZ MMM has been reduced from 1.17% to 0.35%, 0.33% and 0.49% with the PZ/IL1, PZ/IL2 and PZ/IL3 MMM, respectively, leading to a significant improvement in gas separation performance, particularly with the CO2/CH4 separation performance surpassing the 2008 upper bound. Additionally, the PZ/IL MMMs also showed enhancement in gas separation performance for the CO2 - CH4 mix gas (50:50) compared to the non-IL MMMs and the neat polymer membrane. The contribution of this chapter is that it further evidenced the effectiveness of using IL as a interfacial binder to minimize the interfacial defects in MMMs as well as enhance the gas separation performance in both ideal and real conditions.