Abstract
A series of mixed-linker Zeolitic Imidazolate Framework(ZIF)-7–8s constructed from different amounts of benzimidazolate (bIm) and 2-methylimidazolate (mIm) linkers were synthesized in this study. We demonstrated that by carefully controlling the ratios between bIm and mIm linkers, the particle morphology and pore size of ZIF-7–8 can be tailored to selectively adsorb CO2 or SF6 with significantly enhanced uptake capacity. ZIF-7–8 with 90% bIm and 10% mIm linkers showed a CO2 uptake of 1.44 mmol g−1 at 293 K (1 bar) and a CO2/N2 selectivity of over 30. On the other hand, ZIF-7–8 with 26% bIm and 74% mIm linkers had a high SF6 uptake of 2.08 mmol g−1 at 293 K (1 bar) as well as a high SF6/N2 selectivity of over 40. Isosteric enthalpy of adsorption calculations and cyclic adsorption experiments confirmed that both CO2 and SF6 were physisorbed on ZIF-7–8. CO2 was found to adsorb rapidly on ZIF-7–8, with over 80% of the total uptake capacity reached within 30 s. Detailed kinetics analysis concluded that the adsorption of CO2 and SF6 on these ZIF-7–8s could be described by pseudo-second order kinetics and the diffusion was found to be governed by a mixture of different mechanisms. The highly tunable sorption properties, high uptake capacity and high selectivity of ZIF-7–8 render them as interesting candidate sorbents for different greenhouse gases.
Highlights
The emission of greenhouse gases has often been linked to climate change
All of the obtained Zeolitic imidazolate frameworks (ZIFs)-7–8s were found to be crystalline by Powder X-ray diffraction (PXRD) (Fig. 2) and the ben zimidazolate (bIm):mIm linker ratio of each sample was deter mined by 1H NMR spectroscopy
This obser vation showed that the bIm content in the ZIF-7–8s increased noticeably when the bIm:mIm ratio in the synthesis mixture surpassed a certain threshold (i.e. 0.40:3.6), which is in good agreement with previous findings made by Thompson et al [41]
Summary
The emission of greenhouse gases has often been linked to climate change. It is important to reduce the emission of these gases as a way to protect the environment. The bulkier bIm linker found in ZIF-7 limits the accessible porosity of the framework, and modulates the framework flexibility [25,26,27,28,29,30] Such structurally induced changes in ZIF7 has previously been observed upon the adsorption or removal of guest molecules, such as CO2 or H2 [22,25,26,27,28,30], and the two interchanging phases of ZIF-7 were identified and confirmed to contain pores of different sizes. We report how the pore size of mixed-linker ZIF-7–8s can be tuned by carefully adjusting the amount of mIm and bIm linkers in the frameworks, and in turn be used to enhance the selective uptake of certain greenhouse gases such as CO2 and SF6. We performed detailed analysis of the adsorption thermodynamics and kinetics of the greenhouse gas adsorption process
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