Styrene Purification by Guest-Induced Restructuring of Pillar[6]arene

Abstract

The separation of styrene (St) and ethylbenzene (EB) mixtures is important in the chemical industry. Here, we explore the St and EB adsorption selectivity of two pillar-shaped macrocyclic pillar[n]arenes (EtP5 and EtP6; n = 5 and 6). Both crystalline and amorphous EtP6 can capture St from a St-EB mixture with remarkably high selectivity. We show that EtP6 can be used to separate St from a 50:50 v/v St:EB mixture, yielding in a single adsorption cycle St with a purity of >99%. Single-crystal structures, powder X-ray diffraction patterns, and molecular simulations all suggest that this selectivity is due to a guest-induced structural change in EtP6 rather than a simple cavity/pore size effect. This restructuring means that the material "self-heals" upon each recrystallization, and St separation can be carried out over multiple cycles with no loss of performance.

Figure 1

Chemical structures: (a) St and EB; (b) EtP5 and EtP6. The internal cavity diameters of EtP5 and EtP6 are based on the inscribed circle of the regular pentagon and hexagon, respectively.

Figure 2

Single-crystal structures: (a) EB@EtP5; (b) EB@EtP6; and (c) St@EtP6. St is accommodated in 2D extrinsic channels between the pillararenes in St@EtP6.

Figure 3

(a) Time-dependent EtP6α solid–vapor sorption plot for St-EB mixture vapor. (b) Time-dependent PXRD patterns of EtP6α: (I) simulated from single-crystal structure of EB@EtP6; after adsorption of St-EB mixture vapor for (II) 1 h, (III) 2 h, and (IV) 8 h; (V) simulated PXRD pattern from single-crystal structure of St@EtP6. (c) Representation of EtP6α structural changes upon uptake of St-EB mixture vapor.

Figure 4

(a) Time-dependent EB@EtP6α solid–vapor sorption plot for St-EB mixture vapor. (b) Time-dependent PXRD patterns of EB@EtP6: (I) original EB@EtP6; after adsorption of St-EB mixture vapor for (II) 1 h, (III) 2 h and (IV) 8 h; (V) simulated from single-crystal structure of St@EtP6. (c) Time-dependent St@EtP6α solid–vapor sorption plot for St-EB mixture vapor. (d) Time-dependent PXRD patterns of St@EtP6: (I) original St@EtP6; after adsorption of St-EB mixture vapor for (II) 1 h and (III) 8 h; (IV) simulated from single-crystal structure of St@EtP6. (e) Structural representation of the irreversible transformation between St@EtP6 and EB@EtP6 upon uptake of St-EB mixture vapor.

Figure 5

(a) Schematic representation of the general method to obtain highly pure St from a St-EB mixture using EtP6 as the adsorbent and the recycling of EtP6. (b) Relative amount of St and EB in the resultant vapor measured by gas chromatography. (c) Maximum uptake of St and EB in EtP6 for 6 h after the same material is recycled five times.