Acetylene/Ethylene Separation and Solid-State Structural Transformation via [2 + 2] Cycloaddition Reactions in 3D...

Purification of ethylene by removing acetylene from an ethylene/acetylene (99/1 v/v) mixture is a challenging task in the petrochemical industry. Our effort toward finding new porous materials that can selectively uptake acetylene resulted in two 3D metal-organic frameworks. Compound 1, {[Zn(p-pda)(bpee)]} (p-pda = p-phenylenediacetate and bpee = 1,2-bis(4-pyridyl)ethylene) turned out to be a nonporous structure due to the presence of a bulky and rigid aromatic ring in the p-pda ligand. Interestingly, replacement of p-pda with succinic acid yields the microporous framework {[Zn₂(μ₃-OH)(suc)_1.5(bpee)]·CH₃OH·2H₂O} (2; suc = succinate), which offers a 1D channel along the a direction occupied by guest methanol and water molecules. Remarkably, the desolvated framework of 2 shows selective uptake of C₂H₂ over other gas molecules such as C₂H₄, C₂H₆, CO₂ and CH₄ at 293 K. An ideal absorbed solution theory (IAST) study predicts a high selectivity value for acetylene adsorption over the other gas molecules mentioned above. The efficiency of removal of acetylene from ethylene/acetylene mixtures containing 1% acetylene was established through a column breakthrough experiment performed at room temperature. The performance of our material in the purification of ethylene by removing acetylene is comparable with those reported in the literature. In the frameworks of both 1 and 2, the ethylenic double bonds of adjacent bpee linkers are aligned parallel and readily undergo [2 + 2] cycloaddition reactions upon UV-light irradiation to yield {[Zn₂(p-pda)₂(rctt-tpcb)]} (1IR) and {[Zn₄(μ₃-OH)₂(suc)₃(rctt-tpcb)]·2CH₃OH·4H₂O} (2IR), respectively (rctt-tpcb = regio-cis,trans,trans-tetrakis(4-pyridyl)cyclobutane). Photochemical structural transformations of 100% were observed in single-crystal to single-crystal fashions, which were also supported by ¹H NMR spectroscopy. The structures of both 2 and 2IR underwent temperature-dependent reversible structural transformations, which was confirmed by SCXRD and DSC analysis. Selective C₂H₂ uptake of the dehydrated framework of 2IR was also examined, which demonstrates results similar to those of 2.