Tuning the porosity of triangular supramolecular adsorbents for superior haloalkane isomer separations

Abstract

Distillation-free separations of haloalkane isomers represents a persistent challenge for the chemical industry. Several classic molecular sorbents show high selectivity in the context of such separations; however, most suffer from limited tunability or poor stability. Herein, we report the results of a comparative study involving three trianglamine and trianglimine macrocycles as supramolecular adsorbents for the selective separation of halobutane isomers. Methylene-bridged trianglamine, <b>TA</b>, was found to capture preferentially 1-chlorobutane (<b>1-CBU</b>) from a mixture of <b>1-CBU</b> and 2-chlorobutane (<b>2-CBU</b>) with a purity of 98.1%. It also separates 1-bromobutane (<b>1-BBU</b>) from a mixture of <b>1-BBU</b> and 2-bromobutane (<b>2-BBU</b>) with a purity of 96.4%. The observed selectivity is ascribed to the thermodynamic stability of the <b>TA</b>-based host-guest complexes. Based on single crystal X-ray diffraction analyses, a [3]pseudorotaxane structure (<b>2TA</b>⊃<b>1-CBU</b>) is formed between <b>TA</b> and <b>1-CBU</b> that is characterized by an increased level of noncovalent interactions compared to the corresponding [2]pseudorotaxane structure seen for <b>TA</b>⊃<b>2-CBU</b>. We believe that molecular sorbents that rely on specific molecular recognition events, such as the triangular pores detailed here, will prove useful as next generation sorbents in energy-efficient separations.