Topologically guided tuning of Zr-MOF pore structures for highly selective separation of C6 alkane isomers

Hao Wang,Stephanie Jensen,Jeremy Cure,Yves J Chabal,Eugeny V Alexandrov,Yu Han,Junzhong Lin,Qibin Xia,Simon J Teat,Davide M Proserpio,Kui Tan,Xinglong Dong,David H Olson,Junliang Sun,Timo Thonhauser,Qining Wang,Jing Li

doi:10.1038/s41467-018-04152-5

Abstract

As an alternative technology to energy intensive distillations, adsorptive separation by porous solids offers lower energy cost and higher efficiency. Herein we report a topology-directed design and synthesis of a series of Zr-based metal-organic frameworks with optimized pore structure for efficient separation of C6 alkane isomers, a critical step in the petroleum refining process to produce gasoline with high octane rating. Zr6O4(OH)4(bptc)3 adsorbs a large amount of n-hexane but excluding branched isomers. The n-hexane uptake is ~70% higher than that of a benchmark adsorbent, zeolite-5A. A derivative structure, Zr6O4(OH)8(H2O)4(abtc)2, is capable of discriminating all three C6 isomers and yielding a high separation factor for 3-methylpentane over 2,3-dimethylbutane. This property is critical for producing gasoline with further improved quality. Multicomponent breakthrough experiments provide a quantitative measure of the capability of these materials for separation of C6 alkane isomers. A detailed structural analysis reveals the unique topology, connectivity and relationship of these compounds.

Highlights

As an alternative technology to energy intensive distillations, adsorptive separation by porous solids offers lower energy cost and higher efficiency
The former is based on the difference in diffusion rate, or in an ideal scenario, on selective molecular exclusion, which usually results in high selectivity, as illustrated by two well-known examples: zeolite 5A for the separation of linear and branched alkane isomers and chabazite zeolite (CHA) for the separation of propane and propylene[8]
In this work we focus on a specific Metal-organic frameworks (MOFs) family, namely structures built on zirconium and tetratopic carboxylate linkers, for the following reasons: Zr-based metal-organic frameworks (Zr-MOFs) are a subgroup of MOFs that generally possess high chemical, thermal and water/moisture stability as a result of strong Zr–O bonds and robust multinuclear secondary building units (SBUs, usually Zr6 clusters)[34]

Summary

Introduction

As an alternative technology to energy intensive distillations, adsorptive separation by porous solids offers lower energy cost and higher efficiency. Adsorption-based separation of hydrocarbons by porous solids can be divided into two categories according to the separation mechanism: kinetically controlled and thermodynamically controlled process[6,7] The former is based on the difference in diffusion rate, or in an ideal scenario, on selective molecular exclusion (or sieving), which usually results in high selectivity, as illustrated by two well-known examples: zeolite 5A for the separation of linear and branched alkane isomers and chabazite zeolite (CHA) for the separation of propane and propylene[8]. Metal-organic frameworks (MOFs) have been extensively investigated for gas storage and separation because of their high porosity but more importantly, they offer fascinating tunability with respect to their pore size, shape, and surface functionality[10,11,12,13,14] These features make them attractive candidates for energy-efficient separation of hydrocarbons via different mechanisms not achievable by traditional porous solids[7,15,16]. Their performance is comparable and, in some aspects, outperforms the benchmark material zeolite 5A

Methods

Results

Conclusion