Introduction
1,3-Butadiene (C4H6) is one of the basic and important raw material for petrochemical industry, which not only can be used in the production of synthetic rubber, but also in the synthesis of synthetic resins and other organic chemical products, such as tetramethylene sulfone, tetrahydrofuran.1-4It is mainly obtained from the by-product of vapour cracking of naphtha.5 However, this process is usually accompanied by the presence of other C4 components, such as n-butene (n-C4H8), iso-butene (iso-C4H8), n-butane (n-C4H10) and iso-butane (iso-C4H10), which will cause structure transformation of polybutadiene in the preparation of synthetic rubber and affect product quality and even reduce polymerization activity by interrupting the polymerization reaction.6 Hence, the separation of C4H6 from the other C4hydrocarbons is very imperative. In addition, due to the similar boiling point, molecular size and physical properties of C4 hydrocarbons, especially for diolefin and mono-olefins,7 the separation of C4H6 from other C4 hydrocarbons is still remain a particular challenge. At present, the separation of C4H6 from other C4 hydrocarbons is mainly carried out by precise controlled extraction distillation in industry.8However, the extraction distillation usually needs high operation temperature (323 K to 393 K) with more than 110 trays of high towers,9 and consume large amounts of organic solvents. In addition, the polymerization of high reactive C4H6 is inevitable at above high temperature distillation process.10 Therefore, the extraction distillation process is energy intensive and environmental unfriendly process, and it is very important to seek for an efficient and low-cost technology to separate C4H6from C4 hydrocarbons.
Among the existing separation technologies, adsorption separation featuring simple operating conditions, energy saving, and high adsorption accuracy has been proved to be a potential separation technology with broad prospects for gas separation.11-13As an emerging advanced adsorbent, metal-organic frameworks (MOFs) are attracting more and more attention by virtue of their multifarious structural topologies, precise structural determination and tunability of pore surface functionalities.14-16 In recent years, MOFs have been widely applied to various separation applications, including CO2 capture,17propane-propylene separation,18,19 hydrogen storage20and so on.21-25 From the point of view of the adsorption mechanism, the efficient separation of gas mixtures by MOFs adsorbents usually depends on the thermodynamic separation based on interaction force,26,27kinetic separation determined by adsorption rate,28-30molecular sieving separation31,32 and gate-opening flexible separation.33,34 Owing to the similarities of molecular sizes, shapes and properties between C4H6 and other C4 hydrocarbons,35 the potential separation of C4H6 from other C4 hydrocarbons by MOFs adsorbents is still a challenge. The information about physical and chemical properties of C4 hydrocarbons are illustrated in Table S1 . Nowadays, most reported MOF adsorbents used for C4H6separation from other C4 hydrocarbons mainly based on the reinforcing interactions between C4H6 and MOF over other C4 hydrocarbons, however the other C4hydrocarbons could be simultaneously adsorbed to a certain extent,36-38 especially for the separation of mono-olefins and diolefin. The co-adsorption separations will cause the low adsorption selectivity and low separation efficiency.39 As well known, molecular sieving is an ideal efficient separation model and has been well achieved on some olefin/alkane separations.40 However, since the very similar molecular size of C4H6 with other C4hydrocarbons, the adsorbent that can separate C4H6 with other C4hydrocarbons by complete molecular sieving effect have not been reported up to now. Ingeniously, considering the difference that C4H6 has two C=C double bonds, whereas the other C4 hydrocarbons either have one or none C=C double bond, gate-opening flexible MOF hopefully can be deemed as the prospective candidate for the separation of C4H6 to achieve the molecular sieving effect. As a very rare example, Kitagawa et al.41reported a flexible SD-65 MOF that can selectively capture C4H6 and exclude other C4 hydrocarbons. However, the gate-opening pressure for C4H6 is up to 0.6 bar at 298 K. The high gate-opening pressure is disadvantageous for the actual purification and separation of C4H6 and the operating range will be relatively narrow, especially, it is difficult to achieve efficient separation at practical low partial pressure of C4H6.42 Considering this problem, the ideal MOF with gate-opening effect on C4H6/C4 hydrocarbons separation should not only can sensitively capture C4H6 at very low pressure but also can exclude other C4 hydrocarbons even at 1 bar, to realize efficient separation of C4H6 over a wide ranges of operation pressures, especially at low partial pressure of C4H6.
Herein, we adopt a guest induced flexible Mn-bpdc MOF with neat one-dimensional channels for the separation of C4H6 from other four major C4 hydrocarbons, including two mono-olefins and two paraffins. The intrinsic flexibility of Mn-bpdc MOF is confirmed by thermal responded variable temperature X-ray diffraction (VT-XRD) and guest-depended structure transformations, while its gate-opening effects on olefins and paraffins are explored by the adsorption isotherms of C2H4, C3H6, C2H6 and C3H8. Surprisingly, the single-component adsorption isotherms of C4 hydrocarbons with Mn-bpdc demonstrates that Mn-bpdc can specifically and sensitively capture C4H6 with very low gate-opening pressure and exclude for other C4 components even to 1 bar, including n-C4H8,iso-C4H8, n-C4H10 and iso-C4H10. At the same time, the uptake selectivities of C4H6/n-C4H8and C4H6/iso-C4H8in Mn-bpdc exceed the overall reported adsorbents. The four column breakthrough experiments of gas mixtures of C4H6/n-C4H8, C4H6/iso-C4H8, C4H6/n-C4H10and C4H6/iso-C4H10furtherly credibly verify that the efficient dynamic separation effect on Mn-bpdc to separate C4H6 from other C4 components can be achieved. In addition, the Mn-bpdc MOF possesses outstanding water stability, well regeneration and cyclic utilization performance, which comprehensively affirm the great potential of Mn-bpdc adsorbent for the challenging separation of C4H6 from other C4hydrocarbons.