Near-Ideal Xylene Selectivity in Adaptive Molecular Pillar[ n]arene Crystals.

Kecheng Jie,Samantha Y Chong,Ashlea R Hughes,Feihe Huang,Andrew Stephenson,Yujuan Zhou,Marc A Little,Angeles Pulido,Fumiyasu Sakakibara,Tomoki Ogoshi,Andrew I Cooper,Ming Liu,Graeme M Day,Frédéric Blanc

doi:10.1021/jacs.8b02621

Abstract

The energy-efficient separation of alkylaromatic compounds is a major industrial sustainability challenge. The use of selectively porous extended frameworks, such as zeolites or metal–organic frameworks, is one solution to this problem. Here, we studied a flexible molecular material, perethylated pillar[n]arene crystals (n = 5, 6), which can be used to separate C8 alkylaromatic compounds. Pillar[6]arene is shown to separate para-xylene from its structural isomers, meta-xylene and ortho-xylene, with 90% specificity in the solid state. Selectivity is an intrinsic property of the pillar[6]arene host, with the flexible pillar[6]arene cavities adapting during adsorption thus enabling preferential adsorption of para-xylene in the solid state. The flexibility of pillar[6]arene as a solid sorbent is rationalized using molecular conformer searches and crystal structure prediction (CSP) combined with comprehensive characterization by X-ray diffraction and 13C solid-state NMR spectroscopy. The CSP study, which takes into account the structural variability of pillar[6]arene, breaks new ground in its own right and showcases the feasibility of applying CSP methods to understand and ultimately to predict the behavior of soft, adaptive molecular crystals.

Highlights

With the expanding global demand for petrochemical feedstocks, the development of new materials that reduce the environmental impact of chemical processing is important
These xylene isomers are obtained from crude oil by catalytic reforming, by toluene disproportionation, and by the distillation of pyrolysis gasoline.[3]. They act as antiknocking additives in gasoline and they are important chemical feedstocks for phthalic anhydrides and phthalonitriles. pX is the most important isomer: it is primarily used as a feedstock with purity requirement of >99%, for terephthalic acid and dimethyl terephthalate production; these compounds are used to prepare polyester fibers and polyethylene terephthalate (PET) resins for beverage bottles.[4−8] The energyefficient separation of pX from oX and mX with high purity is important in large-scale plastics production
We and other groups found that pillararene crystals can be used for gas storage and hydrocarbon separations.[37−40] For example, we recently reported that a perethylated pillar[6]arene can act as a separation material to purify styrene from a mixture of styrene and ethylbenzene.[39]

Summary

INTRODUCTION

With the expanding global demand for petrochemical feedstocks, the development of new materials that reduce the environmental impact of chemical processing is important. Improving the efficiency of the separation and refining of aromatic hydrocarbons is of particular importance, given the large volumes of these compounds that are used as starting materials in the chemical industry.[1] One of the most challenging separations is that of xylene isomers (ortho, meta and para, hereafter referred to as oX, mX and pX), which was classified by Sholl as one of the “seven chemical separations to change the world”.2. The observed combination of adsorptive separation and guest-induced restructuring is similar to flexible MOFs10,41 but reported here for an adaptive molecular crystal We exploited this selectivity to develop an adsorption/desorption process where pX could be separated from a 1:1:1 mixture of the three xylene isomers with a purity of 99.1%

RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES