Highly Selective, High‐Capacity Separation of o‐Xylene from C₈ Aromatics by a Switching Adsorbent Layered Material

Abstract

Purification of the C8 aromatics (xylenes and ethylbenzene) is particularly challenging because of their similar physical properties. It is also relevant because of their industrial utility. Physisorptive separation of C8 aromatics has long been suggested as an energy efficient solution but no physisorbent has yet combined high selectivity (>5) with high adsorption capacity (>50 wt %). Now a counterintuitive approach to the adsorptive separation of o‐xylene from other C8 aromatics involves the study of a known nonporous layered material, [Co(bipy)2(NCS)2]n (sql‐1‐Co‐NCS), which can reversibly switch to C8 aromatics loaded phases with different switching pressures and kinetics, manifesting benchmark o‐xylene selectivity (SOX/EB≈60) and high saturation capacity (>80 wt %). Structural insight into the observed selectivity and capacity is gained by analysis of the crystal structures of C8 aromatics loaded phases.

Crystal engineering of a rectangular sql coordination network to enable xylenes selectivity over ethylbenzene

“…That different switching pressures and/or adsorption rates were observed in sql-1,3-Co-NCS vs. sql-1-Co-NCS 20 suggested to us that sql-1,3-Co-NCS might be suitable for physisorptive separation of C8 aromatics. Vapour-phase binary mixture separation experiments were conducted on sql-1,3-Co-NCS at 85 C and selectivities were determined by 1 H NMR (Fig.…”

“…The interlayer separation of 5.67 Å is consistent with that of Type I-a bipy-based sql nets. 20 Upon exchange with EtOH, sql-1,3-Co-NCS·2 PX transforms to sql-1,3-Co-NCS·3 EtOH, which exhibits a reduced interlayer separation of 4.70 Å ( Fig. 2 ) that is just above the interlayer separation of guest-free Type I-a bipy-based sql nets.…”

“…19 Unfortunately, these "0D materials" lack the combination of high selectivity (>5) and high working capacity (>50 wt%). 20 Recently, we reported that the previously known coordination network (net) sql-1-Co-NCS can behave as a switching adsorbent layered material (SALMA). This square lattice (sql) topology net was observed to exhibit benchmark OX capacity (>85 wt%) and OX/EB selectivity (e.g.…”

“…S OX/EB > 60). 20 sql-1-Co-NCS exhibits switching behaviour as revealed by Type F-IV isotherms 21 that are induced by different C8 isomers at different switching pressures. 20 We attributed the high working capacity of sql-1-Co-NCS to its ability to switch between closed and open phases in a manner similar to that of clays.…”

“… 20 sql-1-Co-NCS exhibits switching behaviour as revealed by Type F-IV isotherms 21 that are induced by different C8 isomers at different switching pressures. 20 We attributed the high working capacity of sql-1-Co-NCS to its ability to switch between closed and open phases in a manner similar to that of clays. Switching behaviour in sql nets was first reported for the related sql net [Cu(bipy) 2 (BF 4 ) 2 ] n , ELM-11 , which was observed to exhibit switching in the presence of gases such as CH 4 , CO 2 , C 2 H 2 , N 2 , O 2 and n -butane.…”