Tuning the separation properties of zeolitic imidazolate framework core–shell structures via post-synthetic modification

Abstract

The conversion of ZIF-8 into ZIF-7 via post-synthetic modification with benzimidazole has been monitored by quantifying the liberated 2-methylimidazole by chromatography. The reaction kinetics have been adjusted to the shrinking core model, providing the diffusion coefficient of bIm inside the pores and the reaction kinetic constant (2.86 × 10⁻⁷ cm² s⁻¹ and 1.36 × 10⁻⁴ cm s⁻¹, respectively). A wide variety of ZIF-7/8 hybrid core–shell frameworks have been obtained during this reaction. The most promising have been characterized by SEM/TEM, TGA, N₂ and CO₂ adsorption, FTIR and ¹³C NMR, showing features of the coexistence of both phases inside the frameworks. Their structures have also been simulated, providing comparable XRD and adsorption results. The hybrid material has been used as a filler for PBI mixed matrix membranes (MMMs) applied to H₂/CO₂ separation, enhancing the performances of the bare PBI polymer and MMMs containing ZIF-8 or ZIF-7 as a filler, with a maximum H₂ permeability value of 1921 Barrer and a H₂/CO₂ selectivity of 11.8.