Design of olefin-phobic zeolite adsorbents for efficient ethane/ethylene separation

Abstract

Selective ethane adsorption from a mixture of ethane ($C_2H_6$)/ethylene ($C_2H_4$) has emerged as an energy-efficient process for obtaining high-purity ethylene. Some microporous organometallic and organic adsorbents have recently been reported to exhibit unique preferential $C_2H_6$ adsorption behaviors. Conversely, conventional adsorbents such as zeolites are known to preferentially adsorb $C_2H_4$ over $C_2H_6$ (olefin-philic) due to high surface polarity and acidity. Here, we systematically investigated the effects of chemical compositions, silanol defects, and pore topologies (BEA, CHA, and MFI) to develop olefin-phobic zeolites. The results indicated that even the minor presence of Si-O-(Na$^{+}$)-Al (weak Lewis acid) and Si-OH (weak acidic Brønsted acid) groups can undesirably increase the affinity for $C_2H_4$. Consequently, only the defect-free pure-silica zeolites synthesized in fluoride media exhibited high $C_2H_6$/$C_2H_4$ selectivity (1.98–2.25), which were superior to those of Al-containing zeolites (0.26–1.17) and pure-silica zeolites with silanol defects (1.36–1.51). In particular, BEA zeolite exhibited the highest $C_2H_6$/$C_2H_4$ selectivity (2.25), large ethane uptake (2.27 mmol g$^{-1}$) and efficient regeneration at 298 K. It enabled the separation of high-purity $C_2H_4$ (>99.95%) from a mixture of $C_2H_6$/$C_2H_4$ (1/1 v/v) in large productivity (22.98 LSTP L$^{-1}$), which is very promising compared to various state-of-the-art adsorbents.