Improved CO₂ capture performances of ZIF-90 through sequential reduction and lithiation reactions to form a hard/hard structure

Abstract

Post-synthetic functionalization or modification has been regarded as a promising strategy to treat surfaces of adsorbents for their applications in targeted adsorption and separation processes. In this work, a novel microporous adsorbent for carbon capturing was developed via functionalization of zeolitic imidazolate framework-91 (ZIF-91) to generate a hard/hard (metal-oxygen) structure named as lithium-modified ZIF-91 (ZIF-91-OLi compound). To this purpose, the ZIF-91 compound as an intermediate product was achieved by reduction of ZIF-90 in the presence of NaBH₄ as a good reducing agent. Afterwards, acidic hydrogen atoms in the hydroxyl groups of ZIF-91 were exchanged with lithium cations via reaction of n-BuLi compound as an organo lithium agent through an appropriate procedure. In particular, the as-synthesized ZIF-91-OLi operated as an excellent electron-rich center for CO₂ adsorption through trapping the positive carbon centers in the CO₂ molecule. DFT calculations revealed that the presence of lithium over the surface of ZIF-91-OLi adsorbent plays an effective role in double enhancement of CO₂ storage via creating a strong negative charge center at the oxygen atoms of the imidazolate linker as a result of the lithium/hydrogen exchange system. Finally, the selectivity of CO₂/N₂ was investigated at different temperatures, revealing the ZIF-91-OLi as a selective adsorbent for industrial application.

Graphical abstract

Keywords

hard/hard structure / acidic hydrogen / ZIF-91 / carbon capture / ZIF-91-OLi

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Higher Education Press