Supported by the National Science Foundation of China (Nos. 21731002,21975104), researchers at Jinan University led by Prof. Dan Li and Prof. Weigang Lu proposed a new sieving mechanism, namely orthogonal-array dynamic molecular sieving, which enables both fast adsorption-desorption kinetics and large separation capacity as opposed to traditional molecular sieving. A newly invented metal-organic framework (termed JNU-3) with such an underlying sieving mechanism was found to be able to efficiently separate propylene (≥99.5%) in a single adsorption-desorption cycle from an equimolar mixture of propylene/propane. The results were published in Nature entitled “Orthogonal-array dynamic molecular sieving of propylene/propane mixtures” on July 21^st, 2021. The paper can be accessed at: https://www.nature.com/articles/s41586-021-03627-8.

Propylene is one of the most important organic raw materials with an annual output of more than 100 million tons. Currently, the industrial production of propylene is mainly carried out by the steam cracking of propane. However, separating propylene from propane usually comes at a cost of high investment in equipment and huge energy consumption. Traditional molecular sieving has been widely used in many areas including petrochemistry, coal industry, gas separation, and air cleaning, etc. Yet there are some inherent disadvantages with traditional molecular sieves, in particular, difficulty in tuning the pore geometry and slow adsorption-desorption kinetics.

To tackle these issues, researchers at Jinan University proposed a new sieving mechanism, namely orthogonal-array dynamic molecular sieving, and further constructed a metal-organic framework (termed JNU-3) with such an underlying mechanism for propylene/propane separation. JNU-3 is a three-dimensional (3D) framework with a one-dimensional (1D) channel of ca.4.5 Å × 5.3 Å along the crystallographic a axis. Orthogonally-arranged molecular pockets on both sides of the channel are connected through gourd-shaped openings of ca.3.7 Å. The 1D channel facilitates fast gas diffusion, while the pockets selectively capture propylene molecules through the dynamics of the gourd-shaped opening. As a result, 53.5 L of polymer-grade propylene (≥99.5%) can be obtained from an equimolar propylene/propane mixture for 1 kg of JNU-3. in situ single-crystal X-ray diffraction and computational studies further confirmed the dynamic nature of the opening and the proposed propylene/propanesieving mechanism.

The study will provide not only an alternative green and efficient separation for propylene/propane and other important gas molecules, but also a new design strategy for next-generation molecular sieving.

Fig. 1 Schematic representation of tandem and orthogonal sieving mechanisms (left, rigid; right, dynamic)