A method for defect repair of MFI-type zeolite membranes by multivalent ion infiltration

Abstract

MFI-type zeolite membranes are effective for molecular separations, however suffer from non-selective defects. Here we show a means of plugging defects by infiltrating with rigidly bound multivalent ions, and demonstrate improvement to performance as salt rejecting (water selective) desalination membranes. Ion uptake into the MFI-type zeolite powder during exposure to a pH 2 solution containing Fe3+, Al3+, Ca2+ and Mg2+ showed a strong uptake of Fe3+ and Al3+. N2 gas porosimetry showed an increase in micropore proportion, being evidence that adsorbed ions did not enter the intrinsic pores of MFI-type zeolites, instead occupying the larger microporous (grain boundaries) and mesopores. X-ray diffraction (XRD) showed shrinkage in the zeolite crystal, being evidence of loss of monovalent cations within the intrinsic pores supported by the ion uptake results. Zeolite membranes were infiltrated with the solution at 7 MPa. Acid and water leaching revealed Fe3+ was the most strongly incorporated. This was confirmed by energy-dispersive X-ray spectroscopy (EDS) mapping on the surface of the membrane. The practical effect of the defect repair method was demonstrated on four membranes, where salt rejections in a reverse osmosis experiment were consistently improved (e.g. salt rejection increased from 24% to 84%). Further work should consider benefits to other applications of zeolite membranes including gas separation and pervaporation.