Design of Olefin-Phobic Zeolites for Efficient Ethane and Ethylene Separation

Abstract

Selective adsorption of ethane from a mixture of ethane (C 2 H 6 ) and ethylene (C 2 H 4 ) has emerged as an energy-efficient process for obtaining high-purity ethylene. Recently, it has been reported that some novel microporous organometallic and organic adsorbents can preferentially adsorb C 2 H 6 over C 2 H 4 . Conversely, zeolite adsorbents, which are widely used in industry due to their high stability and low material cost, generally prefer C 2 H 4 adsorption ("olefin-philic"). In this study, we carefully investigated the effects of chemical composition (Si/ Al ratio), silanol defect, and pore topology (BEA, CHA, and MFI) of zeolites to develop efficient "olefin-phobic" adsorbents. The results showed that the Si−O−(Na + )−Al (weak Lewis acid) and isolated Si− OH (weak Brønsted acid) groups in the zeolite frameworks undesirably increased the affinity for C 2 H 4 (Lewis base). Consequently, defect-free pure silica zeolites exhibited promising C 2 H 6 /C 2 H 4 selectivities (1.98−2.25), which were superior to those of Al-containing zeolites (0.26) and pure silica zeolites with abundant silanols (1.50), at 298 K and 1 bar. In particular, defect-free pure silica BEA exhibited the highest C 2 H 6 /C 2 H 4 selectivity (2.25), large C 2 H 6 uptake (2.27 mmol g −1 ), and facile regeneration at 298 K. The adsorbent enabled the efficient separation of high-purity C 2 H 4 (>99.95%) from a mixture of C 2 H 6 /C 2 H 4 with high productivity (22.98 L L −1 ). The productivity was comparable or even superior to that of recently reported C 2 H 6 -selective adsorbents. The present results provide hope for the development of efficient, stable, and scalable inorganic adsorbents for the selective C 2 H 6 adsorption.

Fluoride- and Seed-Free Synthesis of Pure-Silica Zeolite Adsorbent and Matrix Using OSDA-Mismatch Approach

“… a Group I comprises several pure-silica zeolites that are typically synthesized hydrothermally in the fluoride media, as reported in the literature. ,− Group II contains pure-silica zeolites that can be hydrothermally synthesized in the alkaline media. − b Abbreviations of OSDAs utilized in this study, along with their corresponding structural formulas, can be found in the Supporting Information. c Si-SOD is used as the sole silica source. …”

“… a Group I comprises several pure-silica zeolites that are typically synthesized hydrothermally in the fluoride media, as reported in the literature. ,− Group II contains pure-silica zeolites that can be hydrothermally synthesized in the alkaline media. − …”

“…Pure-silica zeolites have good alkane/alkene adsorption and separation performance due to their unique microporous structure, lack of acid sites, and high thermal stability. − Notably, Si-CHA exhibits effective adsorption for alkane/alkene separation . Thus, the single-component adsorption isotherms of calcined Si-CHA and Si-CHA-F (synthesized in the fluoride medium) zeolites for propylene and propane are measured at 298 K (Figure a).…”

“…Pure-silica zeolites, which contain well-defined channel systems and/or cavities with molecular-sized pore openings constructed by the conner-sharing SiO 4 , have garnered increasing scientific and industrial attention. It is due to their wide applications as matrixes confining metal species for further catalysis, low-k dielectric materials, and adsorbents for separating alkane/alkene, etc. − Currently, there are 260 framework-type codes (FTCs) approved by the International Zeolite Association-Structure Commission (IZA-SC), while only 58 FTCs can be available in pure-silica forms. − Unfortunately, in most cases, pure-silica zeolites can be synthesized only in the presence of toxic fluoride by conventional hydrothermal synthesis, hindering their further industrial applications. Thus, many efforts have been devoted to synthesizing pure-silica zeolites in the absence of fluorides.…”

Hydrogen bonding to oxygen in siloxane bonds drives liquid phase adsorption of primary alcohols in high-silica zeolites

“…71,79 A similar mechanism involving the interaction of H in alkane C–H bonds with O in siloxane bridges have been proposed to drive the selective adsorption of ethane over ethylene on defect-free, purely-siliceous zeolites. 80 Also, unexpected isotope exchange of oxygen ( 17 O) into zeolite frameworks via equilibration of the zeolite with 17 O enriched water at room temperature 81–83 could be explained by a similar hydrogen bonding mechanism. H-Bonding of the isotope-enriched water to the siloxane bridges could assist in the reversible opening and closure of the siloxane bridges, thus enabling the exchange of framework 16 O with 17 O from water.…”