Zeolite/polyimide mixed-matrix membranes with enhanced natural gas purification performance: Importance of filler structural integrity

Abstract

Mixed-matrix formulation is a promising method to overcome permeability-selectivity trade-off of polymers with incorporation of fillers with well-defined nanopores. The filler properties play a key role in determining the performance of mixed-matrix membrane (MMM). In this work, for the first time, we explored the critical effect of filler structural integrity on separation performance of MMM. Typically, micron as-synthesized, submicron as-synthesized and submicron ground SAPO-34 zeolites were incorporated into 6FDA-DAM polyimide, respectively. The interface compatibility between two phases was ensured by (3-aminopropyl) trimethoxysilane (APTMS) modification on SAPO-34 fillers. It was found that the directly synthesized micron and submicron fillers contributed to enhance the CO2/CH4 separation performance whereas the submicron ground filler lowered the CO2 permeability. Notably, incorporating 30 wt% micron as-synthesized SAPO-34 into 6FDA-DAM enhanced the CO2 permeability by 68% and CO2/CH4 by 37%, surpassing the performance upper-bound. Moreover, the hydrogen bonding between the filler and polymer matrix endowed the membrane with outstanding plasticization resistance with CO2 partial pressure up to 20 bar. Adsorption isotherms and Maxwell model prediction revealed that the filler transport properties could be significantly affected by commonly-used methods of reducing the filler size. A decreased structural integrity was identified in the sequence of micron as-synthesized, submicron as-synthesized and submicron ground fillers. This work highlights the importance of a careful balance between structural integrity and crystal size of filler, and reliable routes to obtain nanofillers with desirable structural integrity as pursuing high-performing MMM toward practical application.