Separation of hexane isomers by introducing “triangular-like and quadrilateral-like channels” in a bcu-type metal-organic framework

Abstract

The separation of hexane isomers is of vital importance to produce high quality gasoline in the petrochemical industry. However, the similar vapor pressure and boiling point of hexane isomers bring great difficulties and challenges in the separation process. Sieving effect, which allowing smaller molecules pass through and preventing others, should be a powerful strategy to solve this problem by making good use of porous materials. Therefore, physical separation by metal-organic framework (MOF) materials appears and becomes a burgeoning separation technique in industry. Due to the weak interaction between hexane isomers with absorbents, it puts forward higher requirements for the accurate design of MOF materials with optimal pore system. To address this issue, a novel MOF [Zn9(tba)9(dabco)3]·12DMA·6MeOH (abbreviation: Zn9(tba)9(dabco)3; H2tba = 4-(1H-tetrazol-5-yl)-benzoic acid; dabco = 1,4-diazabicyclo[2.2.2]octane; DMA = N, N-dimethylacetamide) with bcu network has been designed and synthesized by reticular chemistry strategy. Benefiting from the pre-designed topology and suitable linear ligand H2tba and dabco, the structure of Zn9(tba)9(dabco)3 exhibits two types of channels with triangular-like and quadrilateral-like geometry. Zn9(tba)9(dabco)3 with appropriate channel size and shape displays potential selective adsorption capacity of vapor-phase hexane isomers through sieving effect. Moreover, outstanding gas adsorptive separation properties of Zn9(tba)9(dabco)3 could also be speculated by theoretical ideal adsorbed solution theory (IAST), suggesting Zn9(tba)9(dabco)3 can be regarded as a potential adsorbent material for purification natural gas. Breakthrough experiments show that Zn9(tba)9(dabco)3 is capable of discriminating all four hexane isomers at 298 K, and the corresponding research octane number (RON) of the eluted mixture closes to 95, which is higher than the standard for industrially refined hexane blends (about 83). We speculate that sieving effect and diffusion are a synergetic contributory factor in their elution dynamics, which may be ascribed to temperature-dependent interaction between pore aperture and each isomer. This work presents a typical example for design of efficient MOF absorbents by reticular chemistry strategy.