Tuning 1-hexene/n-hexane adsorption on MOF-74 via constructing Co-Mg bimetallic frameworks

A series of bimetallic CoxMg1-x-MOF-74 (x = 0, 0.12, 0.30, 0.57, 0.78, 1) structures were synthesized by using facile solvothermal systems having different binary metal compositions. All MOF samples were characterized carefully. Both adsorption affinity to n-hexane and 1-hexene and adsorption selectivity of 1-hexene over n-hexane were evaluated by static adsorption test coupled with computer simulation. It is indicated that the Co-Mg bimetallic MOF-74 frameworks can be successfully constructed via using well-designed reactant compositions. All bimetallic MOF-74 retain almost the same crystalline structure but higher porosity as compared to their monometallic samples. In addition, both of the adsorption capacity and selectivity of Co-Mg bimetallic MOF-74 samples are found strongly dependent on their metal compositions. Specifically, Co0.30Mg0.70-MOF-74 exhibits the largest 1-hexene adsorption capacity of 152.7 mg/g and the highest 1-hexene/n-hexane selectivity of 9.74, which are 2.1-2.3 and 4.1-8.9 times higher than those of monometallic Co- or Mg-MOF-74 samples. Such improvement on adsorption ability and selectivity for olefin molecules can be attributed to the incorporation of more stable coordinatively unsaturated sites (CUS) within Mg-MOF-74 framework, leading to the synergetic effect of pore structure evolution as well as higher density of CUS metal sites in bimetallic frameworks. The tunable adsorption affinity to olefin/paraffin on MOF-74 by means of metal modification provides an approach to the efficient separation of olefins and paraffins from liquid hydrocarbon mixtures.