Wall-trapped acetylene tetramer in a metal-organic framework enables kinetic separation of C2H2/C2H4

Abstract

Effective separation of acetylene/ethylene (C2H2/C2H4) mixtures by adsorption is a challenging but green method given their similar molecular characters. An increasing number of metal-organic frameworks (MOFs) have addressed the diffusion-related challenges to achieve the separation, yet almost all of them just focus on diffusion rates and disregard the geometrical arrangement of adsorbed molecules which exerts important effects in boosting the diffusion behaviors. Herein, we now present a paradigm using an ultramicroporous zinc-MOF (Zn-MOF, referred to as 1) with assessable size of 3.4 × 3.3 Å2, as demonstrated by single-crystal and theoretical analysis, which realizes the effective separation of C2H2 from its competitors. Static adsorption isotherms indicate that 1 possesses higher C2H2 uptake of 28.4 cm3 g−1 at 298 K and 0.1 bar, achieving a high IAST selectivity of 265 among the benchmark MOFs. Theoretical calculations reveal that C2H2 molecules are grasped through multiple binding interactions and adsorbed in the pore surface rather than the center of the pore, thus forming C2H2 tetramer through connecting four C2H2 motifs. Further, time-dependent kinetic tests coupled with molecular dynamics confirm the faster diffusion behavior for C2H2, mainly attributing to unimpeded diffusion channel. Column breakthrough tests demonstrate that 1 can easily accomplish exclusive separation of C2H2 from various binary ethane dehydrogenation (EDH) and senary non-oxidative coupling of methane (non-OCM) byproducts. Specially, structurally stable 1 can be readily synthesized utilizing inexpensive raw materials, yielding the cost of only $614 per kilogram. Accordingly, this work proposes a novel strategy of wall-trapped C2H2 tetramer in MOF that can effectively promote the kinetic behavior through available diffusion channel, expecting to offer a new perspective and provide guidance for future research.