Improving ZIF-8 stability in the preparation process of polyimide-based organic solvent nanofiltration membrane

Abstract

Recently, Zeolitic imidazolate framework-8 (ZIF-8) has drawn considerable attention in preparing mixed matrix membranes (MMMs) because of high specific surface area and unique pore structure. However, the structure degradation of ZIF-8 was found in the preparation process of a polyimide (PI)-based MMM due to the protonation effect in an acid environment of polyamide acid (PAA) solution. Herein, a novel strategy was proposed to overcome this obstacle by transferring the coordinating linkers of ZIF-8 structure into carbon skeleton via direct carbonization. As expected, the unique framework of carbonized ZIF-8 (CZIF-8) was fully retained in MMM preparation process. Meanwhile, CZIF-8 provided preferential flow paths for the solvents because of improved porosity and enhanced sorption capacity for the MMMs. As a result, the 10 wt% CZIF-8/PI MMM exhibited a high rejection of >90% to congo red (MW of 696 g mol−1) and outstanding permeances of 16.52, 4.05 and 2.08 L m−2 h−1 bar−1 in water, ethanol and isopropanol, respectively. This method indicates the feasibility of the proposed strategy and effectively enables the ZIF-8 stability for practical applications in an acid environment.