Pore interconnectivity and surface accessibility in stiffened mixed linker MOFs: An investigation using variable energy positron spectroscopy

Abstract

Zeolitic imidazolate framework-8 (ZIF-8) based membranes show poor selectivity for gas separation due to flexibility of the frameworks. Mixed linker frameworks (ZIF-7x-8) membranes are shown to have higher separation selectivity and low permeance of gases. In order to investigate the role of pores' interconnectivity and surface accessibility towards gases’ permeance and selectivity, variable energy positron spectroscopy (VEPS) measurements have been carried out for ZIF-7x-8 (x ​= ​0, 20.6, 52.6, 79.4, 90.4, 97.2 and 100%) membranes. These membranes were deposited using fast current driven synthesis method to produce a stiffened phase (Cm) of ZIF-8. The frameworks have been characterized using XRD, FTIR, Raman spectroscopy and FE-SEM for their crystalline structure, chemical bonding and morphology. Depth dependent Doppler broadening measurements using VEPS show distinct variations in S-E and 3γ/2γ-E profiles of the membranes. The positronium diffusion length determined by fitting of S-E profiles considering a diffusion model shows a drastic reduction in pore interconnectivity and their surface accessibility for x ​= ​20–97%. These modifications explain the reduction in gas permeance of ZIF-7x-8 based membranes. The present study confirms that membranes having x∼20% shows higher selectivity due to their ZIF-8_Cm crystal structure, reasonable pore interconnectivity and limited non-selective grain boundary structure diffusion.