Stretched ZIF-8@GO flake-like fillers via pre-Zn(II)-doping strategy to enhance CO2 permeation in mixed matrix membranes

Graphene oxide (GO) materials have been widely attempted as functional fillers in mixed matrix membranes (MMMs). In virtue of their high-aspect-ratio sheet structure, they are very effective to tune gas diffusion pathway and enhance selectivity. However, the frequently-used GO sheets are often stacking and folding like barriers in MMMs, and accordingly impair gas permeation ability seriously. In order to make GO fillers stretched and dispersed to decrease mass transfer barriers in MMMs, the modification strategy with continuous ZIF-8 cover layer forming on the surface of GO sheets was proposed. In this composite structure, the rigid ZIF-8 layer, acting like an armor suit, would unfold the GO sheets nicely. Afterwards, the MMMs with ZIF-8@GO flake-like fillers in PEBAx are fabricated by solution casting. SEM images revealed that the specified ZIF-8@GO fillers have been synthesized in accordance with the concept; moreover, the stretched and orderly dispersed structure could be retained well in the MMMs. Gas permeation tests demonstrated that both permeability P-CO2 and ideal selectivity alpha(CO2/N2) are enhanced obviously by dredging the pathways for mass transfer. The MMMs containing 20 wt% stretched ZIF-8@GO achieve the best performance, with P-CO2 P up to 136.2 Barrer and alpha(CO2/N2) up to 77.9 increased 66.0% and 60.0% respectively in comparison with pristine PEBAx membrane. Compare with the MMMs containing equivalent pure GO, this permeability is 74.8% higher, while the ideal selectivity is almost the same. On the whole, the composite flake-like fillers, in which GO sheets were stretched and dispersed perfectly by the ZIF-8 armor, could greatly reduce mass transfer barriers in MMMs and consequently enhance CO2 separation performance.