Fine-Tuning of Gate Opening in Hybrid Zeolitic Imidazolate Framework-8: Inelastic Neutron Scattering and Density Functional Theory Study

The gate opening phenomenon observed in zeolitic imidazolate framework-8 (ZIF-8) under gas pressure is attributed to rotation of the 2-methylimidazole (mIm) linker. Gate opening in ZIF-8 has detrimental effects on molecular gas separation selectivity; hence, fine-tuning of the gate opening of ZIF-8 is being explored through controlled linker and metal exchange maintaining the topology. Herein, we present a systematic investigation using in situ inelastic neutron scattering (INS) corroborated with density functional theory (DFT) of fine-tuning of gate opening in hybrid ZIF-8 under N₂ pressure. For this study, hybrid ZIF-8 samples, (Co/Zn)ZIF-8 having Co (∼25 mol %) and ZIF-7-8 having benzimidazole, bIm (∼17 mol %), were compared with ZIF-8. Using INS, we confirmed that mIm linker rotation that is responsible for the gate opening in ZIF-8 is significantly hindered by Co or bIm substitutions. INS and DFT results pinpoint the origin of this hindered linker swinging motion to stronger metal-linker bonding and the modified structural configuration of the linker position. This combined experimental and theoretical approach has provided clear evidence of the existence of two control mechanisms for molecular separation and importantly provides a molecular scale explanation. This is a design principle that could be employed in the development of pressure-sensitive separation techniques.