Adsorption and Separation of Xylene Isomers: CPO-27-Ni vs HKUST-1 vs NaY

Abstract

The separation of xylene isomers is one of the most difficult separations in petrochemistry. Adsorbents have to preferentially adsorb para or meta isomers, but there are only a few zeolites which fulfill the criteria of selectivity and adsorption capacity. In this study, we evaluate two metal− organic frameworks (MOF) with coordinatively unsaturated metal sites (cus), i.e., CPO-27-Ni and HKUST-1, in the adsorption and separation of p-xylene, m-xylene, and o-xylene in the gas phase. The results are compared with those of the zeolite NaY. CPO-27-Ni and HKUST-1 are both ortho-selective adsorbents, but molecular simulations indicate that the reasons for the ortho selectivity are quite different. The CPO-27-Ni structure is intrinsically ortho selective, and the ortho selectivity is further enhanced by electrostatic effects. HKUST-1, on the other hand, is intrinsically (weakly) para selective but the para selectivity is overcompensated by electrostatic effects, which overall leads to a slight preference for the ortho isomer adsorption. The insights in the adsorption behavior of xylenes that are provided by this study provide useful guidelines in the quest for para-selective MOFs materials.

Gas phase adsorption of alkanes, alkenes and aromatics on the sulfone-DUT-5 Metal Organic Framework

“…At lower temperatures, NaY prefers m-xylene while higher temperatures lead to the preferential adsorption of oxylene. This change was also attributed to packing/arrangement effect [41]. A similar mechanism as observed in MIL-47 and MIL-53 is expected to occur in DUT-5 materials, given their similar pore structure.…”

Adsorption and Diffusion of Xylene Isomers on Mesoporous Beta Zeolite

“…The result shows that o-xylene with a dipole moment interacts more strongly with the electric field of Beta zeolite than p-xylene without dipole moment. Thus, the ortho adsorption is further enhanced by electrostatic effects [27]. However, as to m-xylene, the electrostatic interaction seems to be only of secondary importance in the adsorption.…”

Separating mixtures by exploiting molecular packing effects in microporous materials

“…Binding energies of guest molecules with the framework walls or non-framework cations do not solely determine the separation performance. As pointed out by Peralta et al, 40 adsorbents selective to p-xylene are desirable for high productivities; they need to adsorb only B20% of the feed, whereas an adsorbent that rejects p-xylene would have to adsorb 80% of the feed. In current industrial practice the adsorbent used is BaX zeolite, which selectively adsorbs p-xylene.…”