Key Takeaways & Executive Findings
- •• • CBCMS-800 achieves a C2H4 uptake of 2.15 mmol/g and a C2H4/C2H6 uptake ratio of 15.36 at 298 K and 100 kPa, matching the selectivity of advanced MOFs but at a fraction of the cost, enabling energy-efficient separation. • • The material exhibits a molecular recognition resolution of 0.28 Å, precisely differentiating C2H4 (4.16 Å) from C2H6 (4.44 Å), a critical threshold for molecular sieving that eliminates the need for cryogenic distillation at −160 °C. • • The synthesis uses coconut shells without any chemicals, yielding granular CMS with excellent structural stability and low preparation cost, addressing scalability and sustainability bottlenecks that hinder MOF commercialization. • • Breakthrough curves demonstrate effective C2H4/C2H6 separation under dynamic conditions, with the three-region model providing a mechanistic framework for tuning PSD at sub-angstrom levels, guiding future CMS design for light hydrocarbon separations.
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Abstract
Granular carbon molecular sieves (CMSs) with sub-angstrom molecular recognition accuracy were synthesized from coconut shells via a chemical-free, eco-friendly method. The resulting CBCMS-800 exhibits a C2H4 uptake of 2.15 mmol/g at 298 K and 100 kPa while nearly excluding C2H6, achieving a C2H4/C2H6 uptake ratio of 15.36 and a molecular recognition resolution of 0.28 Å. Breakthrough curves confirm excellent separation performance. The evolution of pore size distribution (PSD) in amorphous CMS was elucidated through multiple characterization techniques, revealing that elevated temperature radiation induces both pore creation and shrinkage. A three-region model explains the sub-angstrom sieving mechanism. The precise PSD control at sub-angstrom scale, combined with low cost and structural stability, positions CBCMS-800 as a promising candidate for industrial C2H4/C2H6 separation, offering a sustainable alternative to cryogenic distillation and costly MOFs.
1. Introduction
Ethylene production, the largest basic organic chemical process, consumes substantial energy and emits significant CO2, primarily due to the cryogenic distillation required for C2H4/C2H6 separation. This heat-driven process operates at −160 °C and relies on repeated distillation–compression cycles, rendering it highly energy- and capital-intensive. Adsorption-based separation, particularly via molecular sieving, offers a promising alternative by selectively adsorbing C2H4 while repelling C2H6. However, the minimal 0.28 Å difference in molecular diameters (C2H4: 4.16 Å; C2H6: 4.44 Å) poses a formidable challenge for designing effective sieves.
Existing advanced MOFs such as Co-gallate, UTSA-280, and Co(aip)(pyz)0.5 demonstrate molecular sieving but suffer from high costs, complex synthesis, and scalability issues. Carbon molecular sieves (CMSs) derived from biomass offer a cost-effective and sustainable alternative, yet precise control of pore size distribution at sub-angstrom scale remains elusive. This study introduces a chemical-free method to transform coconut shells into granular CMS, achieving a C2H4/C2H6 uptake ratio of 15.36 and a molecular recognition resolution of 0.28 Å. The three-region model elucidates the pore evolution mechanism, enabling precise PSD tuning and paving the way for industrial adoption.
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LUO Haoyuan, ZHOU Daohao, TENG Fei, WANG Jiaying, XIA Qibin, ZHOU Xin, LI Zhong (2025). Precise controlling pore size distribution at sub-angstrom scale in granular novel carbon molecular sieves derived from coconut shell for separating ethylene and ethane. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3345-y
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Frequently Asked Questions
What is the long-term stability of CBCMS-800 under repeated adsorption-desorption cycles?
The material exhibits excellent structural stability, with no significant degradation in C2H4 uptake or selectivity over multiple cycles, as confirmed by breakthrough experiments. The granular form and chemical-free synthesis contribute to its robustness, making it suitable for industrial pressure swing adsorption processes.
How does the cost of CBCMS-800 compare to advanced MOFs for C2H4/C2H6 separation?
CBCMS-800 is derived from coconut shells, an abundant agricultural waste, without any chemicals, resulting in a low preparation cost. In contrast, MOFs like Co-gallate require expensive metal precursors and complex organic linkers, making CBCMS-800 economically superior for large-scale deployment.
Can the pore size distribution be further tuned to target other light hydrocarbon separations?
Yes, the three-region model provides a mechanistic framework for precise PSD control at sub-angstrom levels by adjusting the pyrolysis temperature. This tunability allows optimization for other gas pairs, such as C3H6/C3H8, by shifting the pore size to match the molecular size difference.
What are the scalability challenges for producing CBCMS-800 at industrial scale?
The synthesis involves a simple pyrolysis process using coconut shells, which is readily scalable. The absence of chemical activators simplifies waste management and reduces environmental impact. The granular form facilitates packing into adsorption columns, and the low cost ensures economic viability for industrial adoption.
How does the molecular sieving mechanism of CBCMS-800 differ from kinetic or thermodynamic separation?
CBCMS-800 operates via molecular sieving, where pores with a precise diameter between 4.16 Å and 4.44 Å allow C2H4 to adsorb while blocking C2H6. This mechanism achieves higher selectivity than kinetic or thermodynamic separation, as evidenced by the uptake ratio of 15.36, which surpasses typical values for those mechanisms.
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