Source: School of Chemistry
Written by: School of Chemistry
Edited by: Wang Dongmei

Recently, Prof. Jie-Peng Zhang’s Group at School of Chemistry, Sun Yat-sen University reported a new evolution in adsorptive separation by using porous coordination polymers (PCPs), and put froward and verified a new separation mechanism, i.e., intermediate-sized molecular sieving (iSMS), which has been published in the top journal of materials science Nature Materials. The first author is Dr. Dong-Dong Zhou, who got PhD degree in 2016 from the School of Chemistry and now become an associate professor, and the corresponding author is Prof. Jie-Peng Zhang. Besides the members in Prof. Jie-Peng Zhang’s group, the cooperators also include Mr. Pin Chen, Prof. Yunfei Du and Dr. Hui Yan from National Supercomputer Centre in Guangzhou.

Separation and purification of chemical mixtures are major energy consumption in the chemical industry. For example, as the monomer of synthetic rubbers, thermoplastics, resins, etc., styrene (ST) was obtained from catalytic dehydrogenation of ethylbenzene (EB). However, the reaction is reversible with unfavourable thermodynamics, there are a lot of unreacted EB, and some by-products, such as toluene (Tol) and benzene (Bz), in the liquid product. In industry, these mixtures are separated by using multiple vacuum distillation towers to obtain polymer-grade ST (99.5%+).

Adsorptive separation by using porous materials instead of convectional distillation separation method is promising to reduce the energy consumption. Molecular sieve with molecular-level uniform pore size can only allow the molecules smaller than the pore sizes to pass through, achieving ideal adsorption selectivity. However, when there are many components with different sizes in the mixture and the target of purification is not the smallest ones, conventional molecular sieve would be ineffective. For example, the size of ST is between EB and Tol/Bz, using molecular sieve could not obtain high-purity styrene by once adsorption.
Prof. Jie-Peng Zhang’s group is dedicating to the design, function and mechanism of PCPs, and has developed several mechanisms/concepts of adsorption and separation, such as “controlling over the flexibility” (J. Am. Chem. Soc. 2008, 130, 6010; Natl. Sci. Rev. 2018, 5, 907), “Hydrophilic channels capture hydrophobic molecules” (Nat. Commun. 2015, 6, 8697), “Controlling over the molecular configuration to tune adsorption selectivity” (Science 2017, 356, 1193), etc. Recently, they put forward a new adsorptive separation mechanism, i.e., intermediate-sized molecular sieve (iSMS), which can only capture the intermediate-sized molecules, solving major needs for separation and purification of styrene. To achieve the special adsorption behaviour, porous materials must possess suitable flexibility, then combine thermodynamic separation mechanism (too small molecules with weak interactions do not have enough adsorption energy to open the flexible framework) and kinetic separation mechanism (the size of large molecules is bigger than the opened pore aperture), to exclude the non-target components. They designed and synthesized a restricted flexible PCP, MAF-41, achieving only capture ST from the mixture of EB/ST/Tol/Bz with the selectivity of 3300. Through once adsorption-desorption cycle, high-purity ST with 99.9%+ can be obtained. Moreover, MAF-41 shows high thermostability (500 oC), water stability (boiling water and solution of pH 3-14), and superhydrophobicity (water contact degree of 153o), which is very useful in practical applications.

What’s more, Prof Morris from the University of St Andrews published a report as news & views about iSMS in the same issue of Nature Materials (http://dx.doi.org/10.1038/s41563-019-0437-x), and highly appraised this work.

This work had received the generous help from Prof. Xiao-Ming Chen. This work was supported by the NSFC (21731007, 21821003, 91622109 and 21701191), Guangdong Pearl River Talents Program (2017BT01C161) and Guangdong Province Key Area R&D Program (2019B010940001).

Link to the paper:http://dx.doi.org/10.1038/s41563-019-0427-z