Improving Ethane/Ethylene Separation Performance under Humid Conditions by Spatially Modified Zeolitic Imidazolate Frameworks

Abstract

Gas separation performances are usually degraded under humid conditions for many crystalline porous materials because of the lack of water stability and/or the competition of water vapor toward the interaction sites (e.g., open metal sites). Zeolitic imidazolate frameworks (ZIFs) are suitable candidates for practical applications in gas separation because of their excellent physical/chemical stabilities. However, the limitation of substituent positions in common ZIFs has prevented extensive pore engineering to improve their separation performance. In a type of gyroidal ZIFs with gie topology, the Schiff base moiety provides additional substituent positions, making it possible to modify the spatial arrangement of hydrophobic methyl groups. Herein, a new gyroidal ZIF, ZnBAIm (H2BAIm = 1,2-bis­(1-(1H-imidazol-4-yl)­ethylidene)­hydrazine), is designed, synthesized, and characterized. The spatially modified ZnBAIm exhibits improved thermal/chemical/mechanical stabilities compared to ZnBIm (H2BIm = 1,2-bis­((5H-imidazol-4-yl)­methylene)­hydrazine). ZnBAIm can remain intact up to about 480 °C in a N2 atmosphere and tolerate harsh treatments (e.g., 5 M NaOH aqueous solution at room temperature for 24 h and 190 MPa high pressure in the presence of water). Moreover, the modified pore and window sizes have improved significantly the ethane/ethylene selectivity and separation performance under humid conditions for ZnBAIm. Breakthrough experiments demonstrate efficient separation of a C2H6/C2H4 (50/50, v/v) binary gas mixture under ambient conditions; more importantly, the C2H6/C2H4 separation performance is unaffected under highly humid conditions (up to 80% RH). The separation performance is attributed to combined thermodynamic (stronger dispersion interaction with C2H6 than with C2H4) and kinetic factors (diffusion), determined by density functional theory calculations and kinetic adsorption study, respectively.

Optimizing the pore space of a robust nickel–organic framework for efficient C2H2/C2H4 separation

“…The productivity of C 2 H 4 in the C 2 H 2 /C 2 H 4 mixture is higher than those of some promising MOFs, such as ZJU-HOF-1 (0.98 mmol g −1 ) 43 and ZnBAIm (0.3 mmol g −1 ). 44 In addition, the breakthrough study was conducted with C 2 H 2 /C 2 H 6 under the same conditions (Fig. 4b).…”

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

“…Such materials may allow the preferential adsorption of C 2 H 6 having a slightly larger polarizability than C 2 H 4 . Recently, some novel microporous organometallic and organic materials, including metal–organic frameworks (MOFs), − zeolite imidazolate frameworks (ZIFs), − hydrogen-bonded organic frameworks (HOFs), − and porous organic cages (POCs), have been reported to preferentially adsorb C 2 H 6 over C 2 H 4 . Although these materials exhibit a wide range of structural and adsorption properties, several important design principles can be drawn for C 2 H 6 -selective adsorbents.…”

“…First, the adsorbents must have no open metal sites (Lewis acids) that can strongly interact with the π-electrons of C 2 H 4 (Lewis base). Second, the presence of Lewis basic moieties, such as aromatics − ,− ,− , and heteroatoms (N, ,− , O, ,,, and F), is often beneficial for improving the C 2 H 6 /C 2 H 4 selectivity. This can be explained by the fact that C 2 H 6 has six slightly polarized C δ− –H δ+ bonds and the basic moieties can interact via weak C–H···π, − ,− , C–H···N, ,,, C–H···O, ,, and C–H···F hydrogen bonds.…”

Highly Robust Microporous Metal‐Organic Frameworks for Efficient Ethylene Purification under Dry and Humid Conditions

“…[27] It should be noted that only a limited number of MOFs can separate C 2 H 6 /C 2 H 4 in the presence of humidity. [28,29] Most reported C 2 H 6 -selective MOFs are sensitive or labile to humidity, leading to a decrease in gas adsorption or structure damage under humid conditions. For instance, the benchmark C 2 H 6 -selective MOF, Fe 2 (O 2 )(dobdc), needs to be operated in a glove box due to the low structural stability under humidity, [4] causing a critical restriction for its industrial applications under moisture conditions.…”