[BMIM][PF₆] Incorporation Doubles CO₂ Selectivity of ZIF-8: Elucidation of Interactions and Their Consequences on Performance

Abstract

Experiments were combined with atomically detailed simulations and density functional theory (DFT) calculations to understand the effect of incorporation of an ionic liquid (IL), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF]), into a metal organic framework (MOF with a zeolitic imidazolate framework), ZIF-8, on the CO separation performance. The interactions between [BMIM][PF] and ZIF-8 were examined in deep detail, and their consequences on CO/CH, CO/N, and CH/N separation have been elucidated by using experimental measurements complemented by DFT calculations and atomically detailed simulations. Results suggest that IL-MOF interactions strongly affect the gas affinity of materials at low pressure, whereas available pore volume plays a key role for gas adsorption at high pressures. Direct interactions between IL and MOF lead to at least a doubling of CO/CH and CO/N selectivities of ZIF-8. These results provide opportunities for rational design and development of IL-incorporated MOFs with exceptional selectivity for target gas separation applications.

Improving Gas Separation Performance of ZIF-8 by [BMIM][BF4] Incorporation: Interactions and Their Consequences on Performance

“…These conclusions also complement the TGA results confirming the presence of interactions between IL and MOF. 22,23 The 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18 3113 cm -1 assigned to ring ν(CH 3 ) sym and ring ν(CH 3 ) asym are two of these bands. The ring ν(CH 3 ) sym exhibits red shifts of -9, -7, and -9 cm -1 on IL 4 /ZIF-8, IL 20 /ZIF-8, and IL 28 /ZIF-8, respectively, and the ring ν(CH 3 ) asym shows a red shift of -16 cm -1 on each IL/ZIF-8 samples.…”

“…Therefore, we decided to study the incorporation of ILs in a different MOF that have saturated metal sites and 6 ] was incorporated into the pores of ZIF-8 to investigate the separation performance of resulting composite. 23 Remarkable improvements in selectivities were found especially for CO 2 /CH 4 and CO 2 /N 2 separations at low pressures. Interactions between IL and MOF were examined thoroughly using various characterization methods combined with DFT calculations and molecular simulations to elucidate the effect of these interactions on the gas separation performances.…”

“…Thus, we infer that the interaction between MOF and IL is the dominant factor especially at low pressures influencing strongly the Q st values of respective gases as we previously discussed. 23 This is why the effect of IL on the gas selectivity becomes dominant at low pressures. However, we note the increase in ∆Q st does not lead to a significant increase in the CH 4 /N 2 selectivity.…”

Recent advances in the synthesis and applications of metal organic frameworks doped with ionic liquids for CO 2 adsorption

“…Kinik and co-workers [148] studied the effect on the CO 2 separation performance of the incorporation of 1-n-butyl-3-methylimidazolium hexafluorophosphate ( 6 ] as confirmed by BET and XRF measurements. On the contrary, the CO 2 uptake per gram of ZIF-8 remarkably increased up to 0.4 bar upon the IL incorporation, due to the creation of new stronger adsorption sites for CO 2 at low pressures.…”

“…Direct interactions between IL and MOF lead to at least a doubling of CO 2 /CH 4 and CO 2 /N 2 selectivities of ZIF-8. Thus [BMIM][PF 6 ] -incorporated ZIF-8 represents a promising material for gas separation applications, especially for CO 2 separation processes, such as natural gas sweetening and removal of CO 2 from power plant flue gas [148].…”