Synthesis and Molecular Sieving Mechanism of Millet-derived Carbon Molecular Sieves for Separation of C4 Olefins

Abstract

Developing advanced carbon molecular sieves (CMS) for sieving C4H6 from C4 olefins is highly desirable for gas separation in the petrochemical industry. However, this remains an immense challenge to transform amorphous biomass-derived carbon precursor into CMS with molecular-sieving function. Herein, millet, a natural granular biomass, is selected as a carbon source, and millet-derived carbon molecular sieves (MCMSs) are successfully synthesized through pressure-pyrolytic carbonization and controllable pyrolysis. A suite of analytical techniques was employed to investigate the evolution of the microdomain structure and the surface chemistry in amorphous carbon derived from millet. It was found that the pore size distribution (PSD) of millet-derived amorphous carbon materials can be tuned at the sub-angstrom level within the pore-sieving range via the proposed method. Consequently, the resulting samples (MCMS-700 and MCMS-800) exhibit selective adsorption of C4H6, while almost completely excluding both i-butene (i-C4H8) and n-butene (n-C4H8), showcasing excellent C4H6 selectivity. Their IAST-predicted selectivity for C4H6/n-C4H8 and C4H6/i-C4H8 exceed 20,000 at 298 K and 1 bar, completely comparable to advanced MOFs with high C4H6 selectivity. The breakthrough experiments demonstrate that the use of the fixed bed packed with MCMS-800 can readily isolate C4H6 from i-C4H8 and n-C4H8, showing excellent dynamic separation performance. The mechanism of isolating C4H6 from i-C4H8 and n-C4H8 is elucidated on the basis of the PSD model for amorphous carbon molecular sieves.