Highly Selective Adsorption of Para-Xylene, Ethylbenzene, and Explicit Exclusion of Ortho-Xylene from Xylene Isomers Using a Pillar-Layered MOF with Tuned Pore Channels

Abstract

Xylene isomer separation is a long-standing challenge due to the nearly identical properties of para-xylene (PX), meta-xylene (MX), ortho-xylene (OX), and ethylbenzene (EB). Here, we report a rationally designed pillar-layered metal-organic framework (MOF), Ni-HDB, incorporating a cylindrical 1,4-diazabicyclo[2.2.2]octane (DABCO) pillar that blocks lateral channels and directs molecular transport through elliptical windows (3.2 x 6.7 & Aring;2). These apertures closely match the dimensions of PX and EB, enabling kinetic sieving. As a result, Ni-HDB exhibits high selectivity for PX and EB, moderate selectivity for MX, and exclusion of OX under ambient conditions. It achieves record liquid-phase selectivities for EB/OX (1943), PX/OX (951), and MX/OX (158), along with high PX and MX adsorption capacities. Comparative studies with isoreticular analogues confirm that DABCO-driven confinement is key to enhancing size-based selectivity. Density functional theory calculations indicate kinetic preference for PX and EB, thermodynamic favorability for MX, and exclusion of OX. Ni-HDB also shows excellent thermal and structural stability, with no performance loss over ten cycles. These results highlight the importance of channel geometry in MOFs and provide a framework for developing next-generation adsorbents for energy-efficient hydrocarbon separations.