Efficient Acid Screening Using Covalent-Organic-Framework Membranes with Angstrom-Sized Pore Channels and Built-in Charge-Assisted Hydrogen Bond Donors

Abstract Acids are extensively used in contemporary industries; however, time-consuming and environmentally unfriendly processes hinder their isolation from wastes containing various ionic species. While membrane technology can overcome these challenges by efficiently extracting analytes of interest, the associated processes typically exhibit inadequate ion-specific selectivity. In this regard, we rationally designed a membrane with uniform angstrom-sized pore channels and built-in charge-assisted hydrogen bond donors that preferentially conduct monobasic acids while exhibiting negligible conductance for other compounds. The ionic selectivity originates from the size screening ability of angstrom-sized channels between protons and other hydrated cations, while the built-in charge-assisted hydrogen bond donor enables the screening of acids by exerting host-guest interactions to varying extent, acting as an anion filter. Furthermore, in contrast to conventional cationic membranes, the strong hydrogen-bonding interactions between the guanidinium moieties and their counter anions resulted in charge transfer from the anions to the membrane skeleton, which switched the effective pore surface charge from positive to negative, thereby facilitating the transport of protons. Ion transport measurements indicated that the resulting membrane exhibited exceptional permeation for protons over other cations and anions of monobasic acids over those of polybasic acids with selectivities up to 4334 and 183, respectively, demonstrating prospects for monobasic acid extraction from waste streams. These findings will aid the design of advanced multifunctional membranes for sophisticated separation.