Introduction
Both acetylene (C2H2) and ethylene (C2H4) are widely used as basic chemical materials in the petrochemical industry. The production of C2H4 is from the fractional distillation of petroleum, which usually coexists with 1% C2H2.1-3 In addition, carbon dioxide (CO2) appears in the production of C2H2 by the thermal cracking process.4,5 Thus, selectively separation of C2H2 from C2H4/C2H2or CO2/C2H2 mixtures is important to meet the requirement of polymer-grade C2H4 and high-purity C2H2 in petrochemical industrials.6,7 However, considering the small difference of molecular dimension of C2H2(3.3 Å×3.3 Å×5.7 Å), C2H4 (3.3 Å×4.2 Å×4.8 Å) and CO2 (3.2 Å×3.3 Å×5.4 Å), as well as their similar physical properties, it is a very challenging to separate C2H2 from C2H4 or CO2.8,9 Although the cryogenic distillation for C2H2 separation is a very mature operation, it requires huge capital and energy input primarily due to the requirement of low temperature operation.10-12 It is necessary to develop a more efficient separation process such as porous materials based adsorptive separation with relatively low energy consumption and favorable regeneration.13,14
Porous materials, such as zeolites, metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs), with porous structures, huge specific surface area, and designable frameworks have received extensive attention in recent years,15-17 as they present great potential to revolutionize some industrial applications, especially in separation, purification, and the storage of gases.18-20An ideal adsorbent for C2H2 separation is expected to be chemically stable, be able to capture trace C2H2 from C2H4 or other feed gas mixtures, and be able to regenerate easily. In recent years, a number of porous materials have been reported for C2H2/C2H4separation through a synergistic approach of pore tuning and functionalization.21,22 In 2016, Xing’s group14 reported the anion-pillared hybrid porous materials SIFSIX-2-Cu-i, with the specific binding sites and suitable pore space to effectively overcome the trade-off effect for C2H2 separation. In 2017, the first case of an ideal molecular sieve for C2H2/C2H4was found called SIFSIX-14-Cu-i (UTSA-200)23 , which with the ultrafine tuning of its pore size (3.4 Å), can not only effectively block C2H4 but also adsorb high amounts of C2H2, thus setting up the benchmarks for both C2H2 adsorption and C2H2/C2H4separation. However, UTSA-200 can also take up a large amount of CO2 or C3H4 at the same conditions,24,25 which significantly restricts its separation performance for C2H2/CO2 and other multicomponent gases mixtures.
Traditional research on porous materials for gas separation mainly focused on those porous structures in the one-dimensional channel (Figure 1a) or cage type pore (Figure 1b). During our exploration of porous materials for C2H2 separation, we realized that one unique class of porous MOFs, which have interlayer pore cavities (Figure 1c)26,27 in their layered structures, had been overlooked. Compared to the traditional pore types, this kind of pore cavity has a narrow pore space, may exhibit multiple host-guest interactions with gas molecules, and thus could be utilized for selective separation of some specific gas component.
Herein, we report an ultramicroporous MOF (Zn2(bpy)(btec))28,29 that incorporates two-dimensional interlayer cavities, which enabled the full entrance of C2H2 and effectively blocked the C2H4 and CO2, thus exhibiting the benchmark C2H2/C2H4and C2H2/CO2 uptake ratios. According to the breakthrough experiments, C2H2 can be directly removed from C2H2/C2H4(1/99, v/v) or C2H2/CO2(50/50, v/v) mixtures and high-purity C2H4 (>99.9999 %), CO2 (>99.999 %), and C2H2 (>98%) can be obtained in the single separation process. More importantly, Zn2(bpy)(btec) can be straightforwardly synthesized at the kilogram scale under room temperature in an aqueous solution. Its good chemical stability, water stability, thermal stability, and cyclic stability are well satisfied requirements of industrial application.