Efficient and selective separation of aqueous sulfate through recognition and precipitation

Abstract

An effective and quick approach to selectively separate sulfate anions from ethanol solution or water/EtOH mixture (1 : 1, v/v) based on the precipitation of the neutral sulfate complex, without the slow crystallization process and by utilizing a positive-charged and low-solubility receptor, is illustrated in this paper. The cationic receptor [HL1]⁺ with the tris(aminomethyl)ethane platform appended with three pyrrolic arms is capable of selectively separating HSO₄⁻ or H₂PO₄⁻ anions in the form of precipitates from EtOH. The mono-protonated tetrapodal receptor [HL2]⁺, based upon a cyclic C4N2 six-member chair-form platform equipped with four pyrrolic moieties, was constructed in situ through anion-templated chemical synthesis from L1 and pyrrole-2-carboxaldehyde. This receptor shows the capability of selectively precipitating sulfate anions in EtOH or ethanol/water solution. A minor structural modification from L1 to L2 through chemical synthesis in situ leads to an increase of selectivity for sulfate binding and precipitation. The solid-state crystal structure of the precipitates [HL2]₂[SO₄] after recrystallization shows the encapsulation of a single sulfate within a cavity embraced by two receptors, thereby forming a supramolecular capsule. This tetrapodal receptor exhibits high affinity for sulfate anions as also revealed by a chloroform–water extraction experiment, and can be recycled after NaOH_(aq) treatment for continuous usage.