Why are propylene and propane so inseparable?

Plastics, home appliances, medical devices, syntHetic fibers, cosmetics... many chemical products in our lives are made ofpropylene. Propylene is one of the world's largest-volume chemicals and an important basic chemical raw material. 

However, in industrial production, propylene and propane are like a pair of good friends who are always together and difficult to separate simply. Propylene and propane are produced by refining petroleum and coexist with each other. They look like "twins", with only two hydrogen atoms different, and are very close in size, with a molecular size difference of only 0.4 Å, equivalent to four hundredths of a nanometer. Because of this, it is extremely challenging to separate propylene and propane accurately and quickly. Nature magazine once pointed out that the development of efficient and energy-saving olefin and alkane separation technology is hailed as one of the seven chemical separation processes that can change the world.

Molecular sieving is the key mechanism to achieve highly selective identification of substances of similar size. Its basic principle is to allow only molecules smaller than the pore size of the adsorbent to enter the pores, while larger molecules are blocked. The theory is perfect, but the reality is cruel. Since narrow pores limit the diffusion of molecules inside, molecular sieving materials have long faced problems such as poor diffusion and mass transfer, low adsorption capacity, and difficulty in desorption, which seriously affects the separation efficiency.

Therefore, Zhejiang University researchers have developed a fast, efficient, and low-carbon molecular sieve material ZU-609 through precise control. This new molecular sieve material allows only propylene molecules to enter and blocks the passage of propane molecules through precise control of the pores, achieving the effect of fast and accurate identification. In order to make the separation process faster, they used a screening channel with "small ends and large middle". There are "isolation piers" at the inlet and outlet of the channel to block propane molecules. After propylene enters, it can quickly pass through the "wide middle" channel. The diffusion coefficient is increased by 1-2 orders of magnitude compared with previous molecular sieve materials.