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Controlling guest conformation for efficient purification of butadiene

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Science  16 Jun 2017:
Vol. 356, Issue 6343, pp. 1193-1196
DOI: 10.1126/science.aam7232

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Selecting against cis conformers

Before 1,3-butadiene can be used to make polymers, it must be separated from similar hydrocarbons in an energy-intensive distillation process. Liao et al. show that a zinc metal—organic framework can accommodate the cis isomer of 1,3-butadiene. It binds less tightly than butane and butene because its π-bond conjugation is broken. They used this preferential desorption to separate 1,3-butadiene with ≥99.5% purity under ambient conditions.

Science, this issue p. 1193

Abstract

Conventional adsorbents preferentially adsorb the small, high-polarity, and unsaturated 1,3-butadiene molecule over the other C4 hydrocarbons from which it must be separated. We show from single-crystal x-ray diffraction and computational simulation that a hydrophilic metal-organic framework, [Zn2(btm)2], where H2btm is bis(5-methyl-1H-1,2,4-triazol-3-yl)methane, has quasi-discrete pores that can induce conformational changes in the flexible guest molecules, weakening 1,3-butadiene adsorption through a large bending energy penalty. In a breakthrough operation at ambient temperature and pressure, this guest conformation–controlling adsorbent eluted 1,3-butadiene first, then butane, butene, and isobutene. Thus, 1,3-butadiene can be efficiently purified (≥99.5%) while avoiding high-temperature conditions that can lead to its undesirable polymerization.

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