Robust Aluminum-Based Metal-Organic Framework Adsorbents with Heteroatom-Functionalized Nanochannels for Hexane Isomer Separation

Metal-organic frameworks (MOFs) have emerged as attractive options for the nonthermal separation of hexane isomers due to their variable channel environments and structural diversity. Here, we report three robust Al-MOFs, KMF-1, MIL-160, and CAU-23, which have nanochannels functionalized with heteroatoms (N, O, and S) for the adsorptive separation of hexane isomers. The adsorption isotherms reveal that these three Al-MOFs can simultaneously adsorb linear, mono-, and dibranched isomers due to their relatively large pore sizes, while showing distinct thermodynamic adsorption behaviors. Significantly, CAU-23, constructed with “S”-atom-functionalized ligands, boasts a notable n-hexane uptake of 3.4 mmol·g^-1, coupled with n-hexane/3-methylpentane and n-hexane/2,2-dimethylbutane uptake ratios of 1.2 and 1.7 at 298 K and a pressure of 19 kPa, respectively, surpassing those of most reported thermodynamically separated MOF materials. Breakthrough measurements demonstrate that CAU-23 effectively separates an n-hexane/3-methylpentane/2,2-dimethylbutane mixture system into linear, mono-, and dibranched components, achieving n-hexane purity levels of up to 99.9%, while providing sufficient retention time between n-hexane, 3-methylpentane, and 2,2-dimethylbutane. The configurational bias Monte Carlo and density functional theory calculations confirmed atomic-scale interactions between heteroatom sites within Al-MOF nanochannels and various isomers, leading to distinct distributions and adsorption characteristics. Moreover, CAU-23 offers a benchmark balance among separation performance, regeneration ability, cost of production, and scalability, highlighting its potential for further research in industrial adsorptive separation of hexane isomers.