Photo-switchable Cu⁺ sites in metal-organic frameworks for adsorptive desulfurization

Photo-switchable metal-organic frameworks (PMOFs) as energy-saving adsorbents for tailorable guest capture show admirable potentials for various applications like adsorptive desulfurization. However, the regulation behavior of most reported PMOFs is based on weak physical interaction, and it is highly desired to introduce specific active sites to satisfy the demand of higher adsorption capacity and selectivity. Herein, for the first time, we prepared the PMOFs, azobenzene-functionalized HKUST-1 (HK-Azo), simultaneously decorated with Cu₂O active sites that possess strong interaction with guest molecules. Due to π-complexation interaction of Cu⁺ with aromatic sulfur compounds, the obtained HK-Azo shows obviously higher adsorption capacity on benzothiophene compared with HKUST-1. Upon ultraviolet (UV) and visible irradiation, azobenzene moieties in the PMOFs can transform their configuration freely and reversibly. Such trans/cis isomerization of azobenzene causes exposure/shelter of Cu₂O active sites, leading to controllable benzothiophene capture. The HK-Azo exhibits the change of benzothiophene uptake up to 29.7% upon trans and cis isomerization, which is obviously higher than HKUST-1 with negligible change. This work may inspire the development of new adsorption process regulated by light for adsorptive desulfurization that is impossible to realize by conventional PMOFs.