Propylene stands as a ubiquitous raw material in our daily lives, serving as a fundamental element in the production of various chemical goods, including plastics, household appliances, medical devices, synthetic fibers, and cosmetics. Undoubtedly, it ranks among the most bulk chemical products globally.
However, there is a formidable challenge in industrial production, where propylene and propane, akin to inseparable companions, coexist, rendering it difficult to separate them. One article in the journal Nature once highlighted that the development of energy-efficient olefin/paraffin separation technology is one of the seven chemical separation processes that can change the world.
Prof. XING Huabin (middle) and his colleagues in the lab
A breakthrough in this area comes from the collaborative efforts of Prof. XING Huabin and Prof. YANG Lifeng’s research team at the College of Chemical and Biological Engineering, Zhejiang University and ZJU-Hangzhou Global Scientific and Technological Innovation Center. Their innovation, molecular sieve ZU-609, features local sieving channels that have molecular sieving gates and rapid diffusion channels, presenting a marked advancement in efficient propylene separation.
The research was published in the journal Science as a First Release version on December 15, 2023, their findings illuminate a key aspect of chemical engineering.
Propylene and propane, extracted from petroleum refinement, look like “twins”, differing only in two hydrogen atoms. They are virtually identical in size, with a minimal difference of 0.4 Å, or 4% of a nanometer. Therefore, it is challenging to rapidly sieve propylene from propane.
Molecular sieving is a key mechanism for achieving high-selectivity recognition of closely-sized substances. Only molecules with the size that is smaller than the pore of the adsorbents could access to the channel, while larger ones are blocked.
It is better read about in theory than carried out in practice. Narrow channels constrain molecular diffusion, and molecular sieving materials have long suffered from the problems such as poor diffusion, low adsorption capacity, and difficult desorption, severely hampering its separation efficiency.
“To accelerate the adsorption kinetics of olefins, industry often increases temperatures to ‘drive away’ gases and make them ‘run’ faster,” said Prof. XING Huabin. However, this method compromises the working capacity of the adsorbent and increases energy consumption. “The heat-driven olefin production process contributes about 1% of the global carbon emissions.”
Achieving rapid mass transfer in confined space has always been the research frontiers. Prof. XING Huabin said that olefins and paraffin can be precisely separated in small vessels in the laboratory, but it may not work on an industrial scale.
Efficient and rapid mass transfer within confined spaces is crucial, and overcoming this challenge is pivotal for enhancing chemical process efficiency on an industrial scale. To this end, the researcher team developed ZU-609, a novel molecular sieve with fast adsorption kinetics. By controlling pore openings, this molecular sieve only adsorb propylene while exclude propane, achieving precise recognition.
In order to enhance olefin diffusion efficiency, the research team devised sieving channels which are “small at both ends and large in the middle”. “Isolation piers” are placed at the entrance and exit of the channel to block propane molecules. Propylene, once in the channel, can quickly pass through the broad middle section, increasing the diffusion coefficient by one to two orders of magnitude compared to previous molecular sieves.
The efficiency of separation is demonstrated by the acquisition of propylene with 99.97% purity from an equimolar propylene-propane mixture through the ZU-609 molecular sieve. Moreover, ZU-609 is easy to be regenerated at room temperature through nitrogen purging or vacuum pumping.
“The molecular sieve can not only capture propylene molecules but also release them quickly, laying the foundation for the development of efficient and low-carbon separation technology of propylene,” Prof. YANG Lifeng said.
“Our research provides new insights into the core issue of microporous diffusion enhancement and sets the stage for the development of low-carbon separation technology,” Prof. XING Huabin concluded. This is also instrumental to the production of ultra-high-purity electronic chemicals.
（From ZJU NEWSROOM）