Efficient CO2 separation in mixed matrix membranes with a hierarchical pore carbon nanostructure

Mixed matrix membranes (MMMs) based on Pebax MH 1657 (Pebax) and "sea-urchin" like hierarchical mesopore-micropore carbon nanotube nanopolyhedras (CNPs) are fabricated for CO2 separation. The membranes have an efficient improvement in CO2 permeability and CO2/CH4 selectivity with the addition of CNPs. The improved gas-separation performance was attributed to the hierarchical mesopore-micropore structure of nanopolyhedras in CNPs and the "sea-urchin" like structure of CNPs. First, abundant mesopores (4.8 nm) with a large pore volume and a connected porous network structure in nanopolyhedras can provide convenient gas transmission pathways to improve the CO2 permeability. Second, micropores (1.1 nm) in nanopolyhedras play a role of molecular sieves and the outstretched CNTs of the "sea-urchin" like structure of CNPs can extend the gas-delivery path, which will enhance the CO2/CH4 selectivity. Therefore, the CO2 permeability and CO2/CH4 selectivity are significantly enhanced. The MMMs with 8 wt% CNP show the highest CO2 permeability of 510.1 Barrer and CO2/CH4 selectivity of 56.8, the CO2 separation performance surpassed the 2008 Robeson upper bound line.